feat(worker): add Prime scheduler abstractions + runner

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

src/ClaudeDo.Worker/Prime/PrimeScheduleSignal.cs

@@ -0,0 +1,29 @@
+namespace ClaudeDo.Worker.Prime;
+
+public sealed class PrimeScheduleSignal : IPrimeScheduleSignal, IDisposable
+{
+    private CancellationTokenSource _cts = new();
+    private readonly object _lock = new();
+
+    public CancellationToken CurrentToken
+    {
+        get { lock (_lock) return _cts.Token; }
+    }
+
+    public void Signal()
+    {
+        CancellationTokenSource old;
+        lock (_lock)
+        {
+            old = _cts;
+            _cts = new CancellationTokenSource();
+        }
+        try { old.Cancel(); } catch { /* already cancelled */ }
+        old.Dispose();
+    }
+
+    public void Dispose()
+    {
+        lock (_lock) _cts.Dispose();
+    }
+}