fix: exempt docs-only PRs from real proof gate

[kevinslin]

Summary

• Exempt external PRs that only change files under docs/ from the real behavior proof merge gate.
• Persist hydrated PR file paths in report frontmatter so the deterministic gate can classify docs-only PRs without relying only on model output.
• Update review prompt/schema guidance and add regression tests for docs-only and mixed docs/source PRs.

Verification

• pnpm run check
• git diff --check

[chatgpt-codex-connector]

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