Migrate task queue from in-memory to BullMQ + Redis

Replace the volatile in-memory Map/Set queue with BullMQ backed by Redis
for persistence across restarts, automatic retry with exponential backoff,
and non-blocking workspace cleanup.

- Two queues: workspace-create (concurrency-limited) and workspace-cleanup
  (with polling instead of sleep-based stop→delete)
- Active workspaces tracked in Redis hash for dedup across restarts
- Stale sweep every 10 minutes catches orphaned workspaces
- Graceful shutdown on SIGTERM/SIGINT
- Replace node-cron with BullMQ repeatable jobs

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

src/workspace-name.ts

@@ -0,0 +1,11 @@
+import type { TaskRequest } from "./types.js";
+
+/** Build a dedup key for a task. */
+export function dedupKey(task: TaskRequest): string {
+  return `${task.taskType}-${task.giteaRepo}-${task.issueNumber}`;
+}
+
+/** Build the workspace name (matches the dedup key). */
+export function workspaceName(task: TaskRequest): string {
+  return `${task.taskType}-${task.giteaRepo}-${task.issueNumber}`;
+}