mandatory authorization check to prevent external contributors from triggering AI agent

[derekmisler]

Summary

Add mandatory authorization check for comment-triggered events to prevent external contributors from abusing AI agent commands.

Changes

• action.yml: Add authorization check step for comment-triggered events

  • Validates author_association when triggered by comments (e.g., /review)
  • Only allows OWNER, MEMBER, COLLABORATOR roles
  • Blocks CONTRIBUTOR, FIRST_TIME_CONTRIBUTOR, NONE, and other external roles
  • Skips for non-comment events (PR triggers, scheduled jobs, workflow_dispatch)

• security/README.md: Update documentation

  • Add authorization as the first security feature
  • Update architecture diagram to show auth check as first step
  • Document scope of auth check (comment events only)

Security Rationale

Comment-triggered workflows (like /review commands) are the main abuse vector:

• External contributors could spam expensive AI operations
• Potential for prompt injection via crafted comments

PR-triggered and scheduled workflows are controlled by the workflow author, so auth is not enforced.

How It Works

Trigger	Auth Check	Result for External User
/review comment	✅ Enforced	❌ Blocked
pull_request event	⏭️ Skipped	✅ Allowed (workflow controls this)
workflow_dispatch	⏭️ Skipped	✅ Allowed
Scheduled job	⏭️ Skipped	✅ Allowed

Backward Compatibility

• Comment-triggered workflows: External contributors will now be blocked
• PR-triggered workflows: No change, works as before
• Scheduled/dispatch workflows: No change, works as before

[derekmisler]

/describe