fix: deduplicate CI watchers and remove task ID from watcher prompts

Prevent duplicate CI completion notifications in paired rooms by checking
for existing active watchers with the same channel+provider+metadata before
creating a new one. Remove Task ID from watcher prompt construction to
avoid router secret redaction (task- IDs contain sk- pattern), passing
EJCLAW_RUNTIME_TASK_ID via env var instead and making cancel_task
auto-resolve when task_id is omitted.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/db.test.ts

@@ -469,9 +469,6 @@ describe('task CRUD', () => {
 Watch target:
 cleanup
 
-Task ID:
-task-cleanup
-
 Check instructions:
 Check the run.
       `.trim(),