feat: add Test Coverage Checker — 6.6

Add automated script that identifies changed source files, checks for
corresponding test changes, and reports coverage gaps.

Acceptance — #124:
  - Identifies changed source files  (git diff --name-only HEAD)
  - Checks for corresponding test changes (source→test file mapping)
  - Reports: code without tests       (lists uncovered sources)
  - Output: coverage gap             (structured text/JSON)

Closes #124
Task: 6.6 — Test Coverage Checker

scripts/coverage_checker.py

@@ -0,0 +1,169 @@
+#!/usr/bin/env python3
+"""
+Test Coverage Checker — 6.6
+
+Identifies changed source files, checks for corresponding test changes,
+and reports code without test coverage.
+
+Usage:
+    python3 scripts/test_coverage_checker.py
+    python3 scripts/test_coverage_checker.py --format json
+    python3 scripts/test_coverage_checker.py --compare HEAD~1  # Compare against a specific ref
+
+Acceptance:
+  - Identifies changed source files   (git diff --name-only HEAD)
+  - Checks for corresponding test changes (matches source→test file mapping)
+  - Reports: code without tests        (lists coverage gaps)
+  - Output: coverage gap              (structured text/JSON)
+"""
+
+import argparse
+import json
+import subprocess
+import sys
+from pathlib import Path
+from typing import List, Tuple, Optional
+
+REPO_ROOT = Path(__file__).resolve().parent.parent
+
+
+def run_git_diff(ref: str = "HEAD") -> List[str]:
+    """Return list of changed file paths relative to given ref."""
+    result = subprocess.run(
+        ["git", "diff", "--name-only", ref],
+        capture_output=True, text=True, cwd=REPO_ROOT
+    )
+    if result.returncode != 0:
+        print(f"ERROR: git diff failed: {result.stderr}")
+        sys.exit(1)
+    return [p for p in result.stdout.splitlines() if p.strip()]
+
+
+def is_source_file(path: str) -> bool:
+    """True if path is a Python source file (not test)."""
+    return path.endswith(".py") and not path.startswith("tests/") and "/test" not in Path(path).name
+
+
+def is_test_file(path: str) -> bool:
+    """True if path is a test file."""
+    if not path.endswith(".py"):
+        return False
+    name = Path(path).name
+    # Test files: test_*.py or *_test.py or in tests/ directory
+    return (name.startswith("test_") or name.endswith("_test.py") or path.startswith("tests/"))
+
+
+def source_to_test_path(src_path: str) -> str:
+    """
+    Map a source file path to its expected test file path.
+    Convention: scripts/<name>.py -> tests/test_<name>.py
+                <module>.py -> tests/test_<module>.py
+    """
+    name = Path(src_path).name
+    stem = Path(name).stem  # without .py
+    # Common mapping: script name -> test_ prefix in tests/
+    test_name = f"test_{stem}.py"
+    return str(Path("tests") / test_name)
+
+
+def test_file_exists() -> bool:
+    """Check if the test file exists in the repo."""
+    return (REPO_ROOT / test_rel).exists()
+
+
+def analyze_coverage(changed_files: List[str]) -> dict:
+    """
+    For each changed source file, check if corresponding test file also changed.
+    Returns structured coverage gap report.
+    """
+    changed_sources = [f for f in changed_files if is_source_file(f)]
+    changed_tests = [f for f in changed_files if is_test_file(f)]
+
+    # Build set of test file paths that changed (relative paths)
+    changed_test_set = set(changed_tests)
+
+    # Build coverage gap
+    uncovered_sources = []
+    covered_sources = []
+    for src in changed_sources:
+        coverage_entry = {"file": src}
+        # Check: does the corresponding test file also appear in changed files?
+        test_rel = source_to_test_path(src)
+        if test_rel in changed_test_set:
+            coverage_entry["status"] = "covered"
+            coverage_entry["test_file"] = test_rel
+            covered_sources.append(coverage_entry)
+        else:
+            coverage_entry["status"] = "missing"
+            coverage_entry["suggested_test"] = test_rel
+            uncovered_sources.append(coverage_entry)
+
+    return {
+        "repo": REPO_ROOT.name,
+        "changed_sources": len(changed_sources),
+        "changed_tests": len(changed_tests),
+        "covered_sources": len(covered_sources),
+        "uncovered_sources": len(uncovered_sources),
+        "coverage_ratio": (
+            len(covered_sources) / len(changed_sources)
+            if changed_sources else 1.0
+        ),
+        "covered": covered_sources,
+        "uncovered": uncovered_sources,
+        "all_changed": changed_files,
+    }
+
+
+def main():
+    parser = argparse.ArgumentParser(description="Test Coverage Checker")
+    parser.add_argument("--format", choices=["text", "json"], default="text",
+                        help="Output format")
+    parser.add_argument("--compare", default="HEAD",
+                        help="Git ref to compare against (default: HEAD)")
+    args = parser.parse_args()
+
+    # Step 1: Identify changed files
+    print(f"Scanning changes vs {args.compare}...")
+    changed_files = run_git_diff(args.compare)
+    if not changed_files:
+        print("No changed files detected.")
+        sys.exit(0)
+
+    # Step 2: Analyze coverage
+    report = analyze_coverage(changed_files)
+
+    if args.format == "json":
+        print(json.dumps(report, indent=2))
+        sys.exit(0)
+
+    # Text output
+    print("=" * 60)
+    print("  TEST COVERAGE CHECKER")
+    print("=" * 60)
+    print(f"  Repository:  {report['repo']}")
+    print(f"  Changed files total: {len(changed_files)}")
+    print(f"  Source files changed: {report['changed_sources']}")
+    print(f"  Test files changed:   {report['changed_tests']}")
+    print()
+    print(f"  Coverage (sources with test changes): {report['coverage_ratio']:.0%}")
+    print(f"    Covered:   {report['covered_sources']} source file(s)")
+    print(f"    Uncovered: {report['uncovered_sources']} source file(s)")
+    print()
+
+    if report["uncovered"]:
+        print("  COVERAGE GAP — Source files without corresponding test changes:")
+        print("  " + "-" * 54)
+        for item in report["uncovered"]:
+            print(f"    {item['file']}")
+            print(f"      Suggested test: {item['suggested_test']}")
+        print()
+        print("  ACTION: Write or update tests for the files above.")
+        sys.exit(1)  # Non-zero exit to flag coverage gap
+    else:
+        print("  All changed source files have corresponding test coverage.")
+
+    print("=" * 60)
+
+
+if __name__ == "__main__":
+    main()

tests/test_coverage_checker.py

@@ -0,0 +1,116 @@
+#!/usr/bin/env python3
+"""Tests for coverage_checker — Issue #124 acceptance validation."""
+
+import subprocess
+import sys
+from pathlib import Path
+
+sys.path.insert(0, str(Path(__file__).parent.parent / "scripts"))
+
+from coverage_checker import (
+    is_source_file,
+    is_test_file,
+    source_to_test_path,
+    analyze_coverage,
+)
+
+
+class TestSourceFileDetection:
+    def test_script_in_scripts_dir(self):
+        assert is_source_file("scripts/freshness.py") is True
+
+    def test_module_in_root(self):
+        assert is_source_file("knowledge_staleness_check.py") is True
+
+    def test_excludes_test_files(self):
+        assert is_source_file("tests/test_freshness.py") is False
+
+    def test_excludes_non_py(self):
+        assert is_source_file("README.md") is False
+
+
+class TestTestFileDetection:
+    def test_test_prefix(self):
+        assert is_test_file("tests/test_freshness.py") is True
+
+    def test_test_suffix(self):
+        assert is_test_file("scripts/freshness_test.py") is True
+
+    def test_regular_py_is_not_test(self):
+        assert is_test_file("scripts/freshness.py") is False
+
+
+class TestSourceToTestMapping:
+    def test_scripts_mapping(self):
+        assert source_to_test_path("scripts/freshness.py") == "tests/test_freshness.py"
+
+    def test_root_module_mapping(self):
+        assert source_to_test_path("knowledge_staleness_check.py") == "tests/test_knowledge_staleness_check.py"
+
+
+class TestAnalyzeCoverage:
+    def test_no_changes(self):
+        report = analyze_coverage([])
+        assert report["changed_sources"] == 0
+        assert report["uncovered_sources"] == 0
+        assert report["coverage_ratio"] == 1.0
+
+    def test_all_covered(self):
+        changed = [
+            "scripts/freshness.py",
+            "tests/test_freshness.py",
+            "scripts/dedup.py",
+            "tests/test_dedup.py",
+        ]
+        report = analyze_coverage(changed)
+        assert report["uncovered_sources"] == 0
+        assert report["covered_sources"] == 2
+
+    def test_gap_detected(self):
+        changed = [
+            "scripts/new_feature.py",
+            "README.md",
+        ]
+        report = analyze_coverage(changed)
+        assert report["uncovered_sources"] == 1
+        assert report["uncovered"][0]["file"] == "scripts/new_feature.py"
+        assert report["uncovered"][0]["suggested_test"] == "tests/test_new_feature.py"
+
+    def test_mixed_coverage(self):
+        changed = [
+            "scripts/covered.py",
+            "tests/test_covered.py",
+            "scripts/uncovered.py",
+        ]
+        report = analyze_coverage(changed)
+        assert report["covered_sources"] == 1
+        assert report["uncovered_sources"] == 1
+
+
+def run_all():
+    t = TestSourceFileDetection()
+    t.test_script_in_scripts_dir()
+    t.test_module_in_root()
+    t.test_excludes_test_files()
+    t.test_excludes_non_py()
+
+    t2 = TestTestFileDetection()
+    t2.test_test_prefix()
+    t2.test_test_suffix()
+    t2.test_regular_py_is_not_test()
+
+    t3 = TestSourceToTestMapping()
+    t3.test_scripts_mapping()
+    t3.test_root_module_mapping()
+
+    t4 = TestAnalyzeCoverage()
+    t4.test_no_changes()
+    t4.test_all_covered()
+    t4.test_gap_detected()
+    t4.test_mixed_coverage()
+
+    print("All 11 tests passed!")
+
+
+if __name__ == "__main__":
+    run_all()