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/Runner/WorktreeManager.cs

@@ -31,8 +31,10 @@ public sealed class WorktreeManager
             throw new InvalidOperationException($"working_dir is not a git repository: {workingDir}");
 
         var baseCommit = await _git.RevParseHeadAsync(workingDir, ct);
-        var shortId = task.Id.Length >= 8 ? task.Id[..8] : task.Id;
-        var branchName = $"claudedo/{shortId}";
+        // Use the full task id (dashes stripped) in the branch name so
+        // two GUIDs sharing an 8-char prefix cannot collide on the same branch.
+        var idForBranch = task.Id.Replace("-", "");
+        var branchName = $"claudedo/{idForBranch}";
         var slug = CommitMessageBuilder.ToSlug(list.Name);
 
         var worktreePath = _cfg.WorktreeRootStrategy.Equals("central", StringComparison.OrdinalIgnoreCase)