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/github.ts

@@ -224,6 +224,37 @@ export class GitHubProvider implements TaskManager {
     }
   }
 
+  async getMergedMRUrl(issueId: number): Promise<string | null> {
+    try {
+      const raw = await this.gh([
+        "pr", "list",
+        "--state", "merged",
+        "--json", "number,title,body,url,mergedAt",
+        "--limit", "20",
+      ]);
+      const prs = JSON.parse(raw) as Array<{
+        number: number;
+        title: string;
+        body: string;
+        url: string;
+        mergedAt: string;
+      }>;
+      
+      const pattern = `#${issueId}`;
+      
+      // Find the most recently merged PR that references this issue
+      // PRs are returned in reverse chronological order by default
+      const matchingPr = prs.find(
+        (pr) =>
+          pr.title.includes(pattern) || (pr.body ?? "").includes(pattern),
+      );
+      
+      return matchingPr?.url ?? null;
+    } catch {
+      return null;
+    }
+  }
+
   async addComment(issueId: number, body: string): Promise<void> {
     // Write body to temp file to preserve newlines
     const tempFile = join(tmpdir(), `devclaw-comment-${Date.now()}.md`);