feat: include PR/MR URL in DEV completion notifications

Adds automatic PR/MR URL detection and inclusion in DEV completion
announcements to provide better visibility into completed work.

Changes:
1. TaskManager interface:
   - Added getMergedMRUrl() method to fetch PR/MR URL for an issue

2. GitHub/GitLab providers:
   - Implemented getMergedMRUrl() to find most recent merged PR/MR
   - Returns URL of PR/MR that references the issue number

3. task_complete tool:
   - Added optional prUrl parameter
   - Auto-detects PR/MR URL if not provided
   - Includes PR URL in announcement with 🔗 prefix
   - Format: 'PR: https://github.com/...'

4. Role templates (dev.md):
   - Updated to instruct workers to include prUrl in task_complete
   - Documents that prUrl is optional (auto-detected as fallback)

Example announcement:
✅ DEV done #101 — Added PR/MR URL to notifications
🔗 PR: https://github.com/user/repo/pull/102
. Moved to QA queue.

Addresses issue #101

lib/task-managers/task-manager.ts

@@ -75,6 +75,9 @@ export interface TaskManager {
   /** Check if any merged MR/PR exists for a specific issue. */
   hasMergedMR(issueId: number): Promise<boolean>;
 
+  /** Get the URL of the most recently merged MR/PR for a specific issue. Returns null if not found. */
+  getMergedMRUrl(issueId: number): Promise<string | null>;
+
   /** Add a comment to an issue. */
   addComment(issueId: number, body: string): Promise<void>;