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>

tests/ClaudeDo.Worker.Tests/Runner/WorktreeManagerTests.cs

@@ -63,7 +63,7 @@ public class WorktreeManagerTests : IDisposable
 
         Assert.NotNull(ctx);
         Assert.True(Directory.Exists(ctx.WorktreePath));
-        Assert.Equal($"claudedo/{task.Id[..8]}", ctx.BranchName);
+        Assert.Equal($"claudedo/{task.Id.Replace("-", "")}", ctx.BranchName);
         Assert.Equal(repo.BaseCommit, ctx.BaseCommit);
 
         var row = await wtRepo.GetByTaskIdAsync(task.Id);