test: verify hardcoded-home path guard from burn/921 branch

Cherry-picks tools/path_guard.py and tests/test_path_guard.py from
burn/921-poka-yoke-hardcoded-paths (commit 5dcb905). All 21 tests pass:

- hardcoded /Users/<name>/ paths are rejected at runtime
- hardcoded /home/<name>/ paths are rejected at runtime
- ~/.hermes/... via expanduser() passes (safe, expanded at runtime)
- valid relative and /tmp/ absolute paths pass
- static scanner catches violations and respects # noqa: hardcoded-path-ok
- comments are skipped by scanner
- directory scanner skips test files and __pycache__

Refs #962

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tools/path_guard.py

@@ -0,0 +1,165 @@
+"""
+tools/path_guard.py — Poka-yoke: Prevent hardcoded home-directory paths.
+
+Validates file paths before tool execution to prevent the latent defect
+of hardcoded paths like /Users/<name>/, /home/<name>/, or ~/ in code
+that gets committed or in runtime arguments.
+
+Usage:
+    from tools.path_guard import validate_path, scan_for_violations
+
+    # Runtime check
+    validate_path("/Users/apayne/.hermes/config")  # noqa: hardcoded-path-ok  # raises PathGuardError
+
+    # Pre-commit scan
+    violations = scan_for_violations("tools/file_tools.py")
+"""
+
+import os
+import re
+from pathlib import Path
+from typing import List, Tuple
+
+# ── Patterns ────────────────────────────────────────────────────────
+
+# Matches hardcoded home-directory paths in string content
+HARDCODED_PATH_PATTERNS = [
+    # /Users/<name>/... (macOS)
+    (re.compile(r"""['"]/(Users)/[\w.-]+/"""), "/Users/<name>/"),
+    # /home/<name>/... (Linux)
+    (re.compile(r"""['"]/home/[\w.-]+/"""), "/home/<name>/"),
+    # Bare ~/... (unexpanded tilde in code — NOT in expanduser() calls)
+    (re.compile(r"""['"]~/[^'"]+['"]"""), "~/..."),  # noqa: hardcoded-path-ok
+    # /root/... (Linux root home)
+    (re.compile(r"""['"]/root/['"]"""), "/root/"),  # noqa: hardcoded-path-ok
+]
+
+# Allowed contexts where ~/ is fine
+SAFE_TILDE_CONTEXTS = re.compile(
+    r"""expanduser|display_path|relpath|os\.path|Path\(|str\(.*home|"""
+    r"""noqa:\s*hardcoded-path-ok|"""  # explicit escape hatch
+    r"""\bprint\(|f['"]|\.format\(|"""  # display/formatting contexts
+    r"""["']~/["']\s*$""",  # just displaying ~/ as prefix
+    re.VERBOSE,
+)
+
+
+class PathGuardError(Exception):
+    """Raised when a hardcoded home-directory path is detected."""
+
+    def __init__(self, path: str, pattern_name: str, suggestion: str):
+        self.path = path
+        self.pattern_name = pattern_name
+        self.suggestion = suggestion
+        super().__init__(
+            f"Hardcoded path detected: {path} matches {pattern_name}. "
+            f"Suggestion: {suggestion}. "
+            f"Use get_hermes_home(), os.environ['HOME'], or annotate with "
+            f"  # noqa: hardcoded-path-ok for legitimate cases."
+        )
+
+
+# ── Runtime Validation ──────────────────────────────────────────────
+
+def validate_path(path: str) -> str:
+    """
+    Validate a file path for hardcoded home directories.
+    Returns the path if valid, raises PathGuardError if not.
+
+    This is meant to be called in tool wrappers (write_file, execute_code)
+    before executing operations with user-supplied paths.
+
+    Note: At runtime, paths from os.path.expanduser() will resolve to
+    /Users/<name>/... — this is expected and allowed. The guard catches
+    paths that were LITERALLY hardcoded in source code or tool arguments
+    that look like they came from a different machine (e.g., a path
+    containing a different username than the current user).
+    """
+    if not path or not isinstance(path, str):
+        return path
+
+    # At runtime, expanded paths matching current HOME are fine
+    home = os.environ.get("HOME", "")
+    if home and path.startswith(home):
+        return path
+
+    # Check for hardcoded /Users/<name>/ (macOS) — but not current user
+    if re.match(r"^/Users/[\w.-]+/", path):
+        raise PathGuardError(
+            path, "/Users/<name>/",
+            f"Use $HOME or os.path.expanduser('~') instead. "
+            f"Got: {path}"
+        )
+
+    # Check for hardcoded /home/<name>/ (Linux)
+    if re.match(r"^/home/[\w.-]+/", path):
+        raise PathGuardError(
+            path, "/home/<name>/",
+            f"Use $HOME or os.path.expanduser('~') instead. "
+            f"Got: {path}"
+        )
+
+    return path
+
+
+def validate_tool_paths(paths: list) -> list:
+    """
+    Validate multiple paths (e.g., from tool arguments).
+    Returns validated list. Raises PathGuardError on first violation.
+    """
+    return [validate_path(p) for p in paths if isinstance(p, str)]
+
+
+# ── File Scanning (Pre-commit / CI) ────────────────────────────────
+
+def scan_file_for_violations(filepath: str) -> List[Tuple[int, str, str, str]]:
+    """
+    Scan a Python file for hardcoded home-directory path patterns.
+    Returns list of (line_number, line_content, pattern_name, suggestion).
+    """
+    violations = []
+    try:
+        with open(filepath) as f:
+            for lineno, line in enumerate(f, 1):
+                # Skip comments and noqa lines
+                stripped = line.strip()
+                if stripped.startswith("#"):
+                    continue
+                if "noqa: hardcoded-path-ok" in line:
+                    continue
+
+                for pattern, name in HARDCODED_PATH_PATTERNS:
+                    if pattern.search(line):
+                        # Special case: ~/ in expanduser/display context is OK
+                        if name == "~/..." and SAFE_TILDE_CONTEXTS.search(line):  # noqa: hardcoded-path-ok
+                            continue
+                        violations.append((lineno, line.rstrip(), name,
+                                         f"Use get_hermes_home(), os.environ['HOME'], or add # noqa: hardcoded-path-ok"))
+    except (IOError, UnicodeDecodeError):
+        pass
+    return violations
+
+
+def scan_directory(root: str, extensions: tuple = (".py",)) -> List[Tuple[str, List]]:
+    """
+    Scan a directory tree for hardcoded path violations.
+    Returns list of (filepath, violations) tuples.
+    """
+    results = []
+    for dirpath, _, filenames in os.walk(root):
+        # Skip hidden dirs, __pycache__, venv, test dirs
+        skip_dirs = {"__pycache__", ".git", "venv", "node_modules", ".hermes"}
+        if any(s in dirpath for s in skip_dirs):
+            continue
+
+        for fname in filenames:
+            if not fname.endswith(extensions):
+                continue
+            # Skip test files (they may legitimately have paths)
+            if fname.startswith("test_") or "/tests/" in dirpath:
+                continue
+            fpath = os.path.join(dirpath, fname)
+            violations = scan_file_for_violations(fpath)
+            if violations:
+                results.append((fpath, violations))
+    return results