fix(worker): address concurrency, cancellation, and resource issues

- claude process: run stdout/stderr reads without ct; rely on
  kill-on-cancel closing the pipes to unblock them — previously
  ReadLineAsync(ct) could hang, stalling task slots and shutdown
- task runner: terminal db writes (task_runs, MarkDone, MarkFailed,
  SetLogPath) now use CancellationToken.None; RunOnceAsync catches
  OCE and finalizes the run row so ContinueAsync can resume
- task repository: GetNextQueuedAgentTaskAsync is now a single
  UPDATE ... RETURNING statement — closes TOCTOU window where two
  loop iterations could dispatch the same queued task
- queue service: dispose CancellationTokenSource in slot-completion
  ContinueWith to stop leaking wait handles
- git service: register ct.Kill(processTree), drain reads without ct,
  always reap via WaitForExitAsync(None) — no more git zombies on
  cancelled worktree ops
- worktree manager: branch name uses full task id (dashes stripped)
  instead of 8-char prefix, eliminating collision risk

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/ClaudeDo.Worker/Services/QueueService.cs

@@ -71,6 +71,7 @@ public sealed class QueueService : BackgroundService
             _ = RunInSlotAsync(task, "override", cts.Token).ContinueWith(_ =>
             {
                 lock (_lock) { _overrideSlot = null; }
+                cts.Dispose();
             }, TaskScheduler.Default);
         }
     }
@@ -94,6 +95,7 @@ public sealed class QueueService : BackgroundService
             _ = RunContinueInSlotAsync(taskId, followUpPrompt, cts.Token).ContinueWith(_ =>
             {
                 lock (_lock) { _overrideSlot = null; }
+                cts.Dispose();
             }, TaskScheduler.Default);
         }
 
@@ -155,6 +157,7 @@ public sealed class QueueService : BackgroundService
                     _ = RunInSlotAsync(task, "queue", cts.Token).ContinueWith(_ =>
                     {
                         lock (_lock) { _queueSlot = null; }
+                        cts.Dispose();
                         WakeQueue(); // Check for next task immediately.
                     }, TaskScheduler.Default);
                 }