docs: clarify CI triggers for Claude commits

[tarunag10]

Summary

• Add a FAQ entry explaining why downstream CI may not run after Claude pushes commits as github-actions[bot].
• Document use_commit_signing: true with id-token: write so Claude can create commits through the Claude GitHub App token path.
• Warn not to pass the default github_token for this setup, note the workflow-file matching caveat, and link to the existing Commit Signing docs.

Verification

• pnpm dlx prettier@3.5.3 --check docs/faq.md
• git diff --check

Closes #319