refactor(worker/queue): split queue waker and picker, auto-wake on enqueue

Slice 3 of the worker state and queue consolidation refactor.

- Add IQueueWaker / QueueWaker (singleton holding the wake semaphore).
- Add IQueuePicker / QueuePicker; raw SQL UPDATE...RETURNING moves out of
  TaskRepository.GetNextQueuedAgentTaskAsync (deleted) and now also filters
  on blocked_by_task_id IS NULL and writes started_at on claim.
- TaskStateService takes IQueueWaker directly; the Func<QueueService>
  indirection is gone. State transitions to Queued auto-wake the dispatcher.
- QueueService waits via the shared waker and dispatches via the picker.
- Drop explicit _queue.WakeQueue() calls in WorkerHub.QueuePlanningSubtasksAsync
  and ExternalMcpService.AddTask. The hub WakeQueue endpoint stays for
  diagnostics, delegating to _waker.Wake().
- Migrate tests; pre-existing flaky AppSettings/ExternalMcp tests untouched.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/ClaudeDo.Worker/Queue/QueuePicker.cs

@@ -0,0 +1,52 @@
+using ClaudeDo.Data;
+using ClaudeDo.Data.Models;
+using Microsoft.EntityFrameworkCore;
+
+namespace ClaudeDo.Worker.Queue;
+
+public sealed class QueuePicker : IQueuePicker
+{
+    private readonly IDbContextFactory<ClaudeDoDbContext> _dbFactory;
+
+    public QueuePicker(IDbContextFactory<ClaudeDoDbContext> dbFactory)
+        => _dbFactory = dbFactory;
+
+    public async Task<TaskEntity?> ClaimNextAsync(DateTime now, CancellationToken ct)
+    {
+        // Atomic queue claim: UPDATE + RETURNING in a single statement prevents TOCTOU races.
+        // Raw SQL because EF cannot express UPDATE...RETURNING.
+        // Eligible task must be Queued, unblocked, due (or unscheduled), and tagged 'agent'
+        // either directly or via its list. EF SQLite stores DateTime as
+        // "yyyy-MM-dd HH:mm:ss.fffffff" — same format used here for comparison.
+        await using var ctx = await _dbFactory.CreateDbContextAsync(ct);
+        var nowStr = now.ToUniversalTime().ToString("yyyy-MM-dd HH:mm:ss.fffffff");
+        var startedAtStr = DateTime.UtcNow.ToString("yyyy-MM-dd HH:mm:ss.fffffff");
+
+        var rows = await ctx.Tasks.FromSqlRaw("""
+            UPDATE tasks SET status = 'running', started_at = {1}
+            WHERE id = (
+                SELECT t.id FROM tasks t
+                WHERE t.status = 'queued'
+                  AND t.blocked_by_task_id IS NULL
+                  AND (t.scheduled_for IS NULL OR t.scheduled_for <= {0})
+                  AND (
+                      EXISTS (
+                          SELECT 1 FROM task_tags tt
+                          JOIN tags tg ON tg.id = tt.tag_id
+                          WHERE tt.task_id = t.id AND tg.name = 'agent'
+                      )
+                      OR EXISTS (
+                          SELECT 1 FROM list_tags lt
+                          JOIN tags tg ON tg.id = lt.tag_id
+                          WHERE lt.list_id = t.list_id AND tg.name = 'agent'
+                      )
+                  )
+                ORDER BY t.sort_order ASC, t.created_at ASC
+                LIMIT 1
+            )
+            RETURNING *
+            """, nowStr, startedAtStr).ToListAsync(ct);
+
+        return rows.FirstOrDefault();
+    }
+}