docs: document Forgejo event behavior in workflow and trigger.ts

Add inline comments explaining:
- Forgejo never fires pull_request_review (sends pull_request_comment)
- Why workflow if-condition cannot gate on @claude for these events
  (payload fields are empty, confirmed by run 12084 being skipped)
- The @claude check is handled in trigger.ts via payload inspection

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.forgejo/workflows/claude.yml

@@ -26,7 +26,13 @@ jobs:
     #     despite action being 'labeled'). Let all through; trigger.ts validates.
     if: >-
       (github.event_name == 'issue_comment' && contains(github.event.comment.body, '@claude')) ||
+      # Forgejo collapses both pull_request_review_comment AND pull_request_review
+      # into pull_request_comment. We cannot gate on @claude here because Forgejo's
+      # webhook payload does not populate github.event.comment.body or
+      # github.event.review.body for these events (tested: run 12084 was skipped).
+      # The @claude check is handled in trigger.ts which reads the actual payload.
       (github.event_name == 'pull_request_review_comment' || github.event_name == 'pull_request_comment') ||
+      # This condition only fires on GitHub (Forgejo never sends pull_request_review).
       (github.event_name == 'pull_request_review' && contains(github.event.review.body, '@claude')) ||
       (github.event_name == 'issues')
     runs-on: grunt