Add reusable GitHub Action for v0.3.0

[benzzy1287]

Summary

• add a reusable root composite Action with strict Git-tracked auditing
• write privacy-safe JSON and Markdown CI summaries plus a v1 SHA-256 receipt
• keep uploads explicit and limited to generated .codex-proof/ files
• add a structured maintainer feedback form and bilingual v0.3.0 docs

Verification

• 15 Python standard-library unit tests pass
• Plugin and Skill validators pass
• strict Git-tracked privacy audit passes with 0 errors and 0 warnings
• v1 receipt generation and verification pass for all 16 tracked files
• local execution of all four composite Action steps passes

Compatibility and privacy

The receipt schema remains codex-proofkit-receipt-v1; existing CLI commands and exit behavior remain compatible. The CLI and composite Action add no credentials, telemetry, subscription dependency, source upload, or network call.