feat: Poka-yoke — prevent hardcoded home-directory paths (closes #921)

Three-layer defense against latent /Users/<name>/ and ~/ path defects:

1. Runtime guard (tools/path_guard.py):
   - validate_path() catches /Users/<name>/, /home/<name>/ in tool args
   - Allows current HOME prefix (expanduser output is safe)
   - Wired into write_file_tool() and execute_code()

2. Pre-commit hook (hooks/pre-commit-path-guard.py):
   - Scans staged .py files for hardcoded path patterns
   - Blocks commit with actionable error message
   - # noqa: hardcoded-path-ok escape hatch for legitimate cases

3. CI lint (scripts/lint_hardcoded_paths.py):
   - Scans directory tree for violations
   - --fix flag shows remediation suggestions
   - Skips test dirs, __pycache__, venv

4. 21 tests (tests/test_path_guard.py):
   - Runtime validation (valid/invalid paths, batch, edge cases)
   - Static scanning (clean files, violations, noqa, comments)
   - Directory scanning (tree traversal, skip rules)

Existing violations annotated with # noqa: hardcoded-path-ok where
legitimate (config defaults, display strings, test fixtures, skills).

tools/code_execution_tool.py

@@ -916,6 +916,25 @@ def execute_code(
     if not code or not code.strip():
         return tool_error("No code provided.")
 
+    # Poka-yoke: scan code for hardcoded home-directory paths
+    try:
+        from tools.path_guard import scan_file_for_violations
+        import tempfile as _tf
+        _tmp = _tf.NamedTemporaryFile(mode="w", suffix=".py", delete=False)
+        _tmp.write(code)
+        _tmp.close()
+        _violations = scan_file_for_violations(_tmp.name)
+        os.unlink(_tmp.name)
+        if _violations:
+            _msgs = [f"Line {ln}: {line[:60]} ({pat})" for ln, line, pat, _ in _violations]
+            return tool_error(
+                f"Code contains hardcoded home-directory path(s):\n  " +
+                "\n  ".join(_msgs) +
+                "\nUse os.environ['HOME'], os.path.expanduser('~'), or get_hermes_home()."
+            )
+    except Exception:
+        pass  # Guard is best-effort; don't block execution on scan failure
+
     # Dispatch: remote backends use file-based RPC, local uses UDS
     from tools.terminal_tool import _get_env_config
     env_type = _get_env_config()["env_type"]