fix: address Copilot PR review comments on agent-governance skill

- Use context manager and path validation for file reading example
- Block directory traversal attacks with os.path.realpath check
- Update terminology: whitelist/blacklist -> allowlist/blocklist

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

skills/agent-governance/SKILL.md

@@ -499,7 +499,12 @@ policy = GovernancePolicy(
 @govern(policy)
 async def read_file(path: str) -> str:
     """Read file contents — governed."""
-    return open(path).read()
+    import os
+    safe_path = os.path.realpath(path)
+    if not safe_path.startswith(os.path.realpath(".")):
+        raise ValueError("Path traversal blocked by governance")
+    with open(safe_path) as f:
+        return f.read()
 ```
 
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