feat: add time-based comment filtering to tag mode (#512)

Implement time-based filtering for GitHub comments and reviews to prevent
malicious actors from editing existing comments after Claude is triggered
to inject harmful content.

Changes:
- Add updatedAt and lastEditedAt fields to GraphQL queries
- Update GitHubComment and GitHubReview types with timestamp fields
- Implement filterCommentsToTriggerTime() and filterReviewsToTriggerTime()
- Add extractTriggerTimestamp() to extract trigger time from webhooks
- Update tag and review modes to pass trigger timestamp to data fetcher

Security benefits:
- Prevents comment injection attacks via post-trigger edits
- Maintains chronological integrity of conversation context
- Ensures only comments in their final state before trigger are processed
- Backward compatible with graceful degradation

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-authored-by: Claude <noreply@anthropic.com>

src/github/types.ts

@@ -10,6 +10,8 @@ export type GitHubComment = {
   body: string;
   author: GitHubAuthor;
   createdAt: string;
+  updatedAt?: string;
+  lastEditedAt?: string;
   isMinimized?: boolean;
 };
 
@@ -41,6 +43,8 @@ export type GitHubReview = {
   body: string;
   state: string;
   submittedAt: string;
+  updatedAt?: string;
+  lastEditedAt?: string;
   comments: {
     nodes: GitHubReviewComment[];
   };