fix: prevent TOCTOU race condition on issue/PR body edits (#710)

Add trigger-time validation for issue/PR body content to prevent attackers
from exploiting a race condition where they edit the body between when an
authorized user triggers @claude and when Claude processes the request.

The existing filterCommentsToTriggerTime() already protected comments -
this extends the same pattern to the main issue/PR body via isBodySafeToUse().

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

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

src/github/types.ts

@@ -58,6 +58,8 @@ export type GitHubPullRequest = {
   headRefName: string;
   headRefOid: string;
   createdAt: string;
+  updatedAt?: string;
+  lastEditedAt?: string;
   additions: number;
   deletions: number;
   state: string;
@@ -83,6 +85,8 @@ export type GitHubIssue = {
   body: string;
   author: GitHubAuthor;
   createdAt: string;
+  updatedAt?: string;
+  lastEditedAt?: string;
   state: string;
   comments: {
     nodes: GitHubComment[];