custom-security-scan-instructions silently ignores documented relative paths

[rplusq]

Expected behavior

The documented relative path works from the caller repository.

custom-security-scan-instructions: .github/custom-security-categories.txt

Current behavior

The composite action changes its working directory to github.action_path before it starts claudecode/github_action_audit.py. The Python process checks Path(scan_file).exists() and opens the input without resolving it against REPO_PATH or GITHUB_WORKSPACE. A relative caller path therefore resolves inside the action repository and is silently ignored.

Minimal reproduction

1. Add .github/custom-security-categories.txt to the caller repository.
2. Pass that relative path as shown in docs/custom-security-scan-instructions.md.
3. Run the action.
4. The log does not contain Loaded custom security scan instructions.

An absolute ${{ github.workspace }}/.github/custom-security-categories.txt path works.

Either resolve relative input paths against REPO_PATH, or update the documentation and examples to require ${{ github.workspace }}. Failing when a nonempty configured path does not exist would also prevent silent loss of the security policy.