fix(background-agent): fix 3 memory leaks in task lifecycle management

H3: cancelTask(skipNotification=true) now schedules task removal.
Previously the early return path skipped cleanup, leaking task objects
in this.tasks Map permanently. Extracted scheduleTaskRemoval() helper
called from both skipNotification and normal paths.

H2: Per-task completion cleanup timer decoupled from allComplete check.
Previously cleanup timer only ran when ALL sibling tasks completed. Now
each finished task gets its own removal timer regardless of siblings.

H1+C2: TaskHistory.clearAll() added and wired into shutdown(). Added
clearSession() calls on session error/deletion and prune cycles.
taskHistory was the only data structure missed by shutdown().

Tests: 10 pass (3 cancel + 3 completion + 4 history)

src/features/background-agent/task-history.ts

@@ -54,6 +54,10 @@ export class TaskHistory {
     this.entries.delete(parentSessionID)
   }
 
+  clearAll(): void {
+    this.entries.clear()
+  }
+
   formatForCompaction(parentSessionID: string): string | null {
     const list = this.getByParentSession(parentSessionID)
     if (list.length === 0) return null