Derive trigger timestamps for issues and pull_request events

[ashwin-ant]

Summary

• Comment/review filtering by trigger time already applied to comment- and review-triggered runs, but extractTriggerTimestamp returned undefined for issues and pull_request events, so runs triggered by an issue being opened/labeled/assigned or a PR event included every comment and review regardless of when it was posted. This derives a trigger timestamp for those events too, so filtering behaves consistently across event types (title/body were already handled).
• issues opened / pull_request opened use created_at; other actions use the payload's updated_at.
• For issues labeled / assigned, the payload doesn't carry the time of the label/assignment itself (and it doesn't bump updated_at), so a new resolveTriggerTimestamp() looks up the matching entry in the issue's event history and uses its created_at, falling back to the payload timestamps if the lookup fails. A matched event older than the payload snapshot is treated as stale and ignored.
• prepareTagMode now awaits resolveTriggerTimestamp instead of calling the sync extractor.

Test plan

• bun test (821 pass) — new coverage for issues opened/labeled/assigned, pull_request opened/synchronize/edited, the event-history lookup (match, no match, API failure, stale event), and comment filtering against the resolved timestamp
• bun run typecheck
• bun run format:check

[claude]

Warning

This review may be incomplete: some analysis steps could not run due to a temporary API capacity limit.