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.Data/Repositories/TaskRepository.cs

@@ -174,26 +174,36 @@ public sealed class TaskRepository
 
     public async Task<TaskEntity?> GetNextQueuedAgentTaskAsync(DateTime now, CancellationToken ct = default)
     {
+        // Atomically claim the next queued agent task: the UPDATE flips its
+        // status to 'running' in the same statement that returns its row,
+        // eliminating the TOCTOU gap where two queue-loop iterations could
+        // both select the same queued task before either marked it running.
+        // The caller is responsible for populating started_at shortly after.
         await using var conn = _factory.Open();
         await using var cmd = conn.CreateCommand();
         cmd.CommandText = """
-            SELECT t.id, t.list_id, t.title, t.description, t.status, t.scheduled_for,
-                   t.result, t.log_path, t.created_at, t.started_at, t.finished_at, t.commit_type,
-                   t.model, t.system_prompt, t.agent_path
-            FROM tasks t
-            WHERE t.status = 'queued'
-              AND (t.scheduled_for IS NULL OR t.scheduled_for <= @now)
-              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'
-                  UNION
-                  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.created_at ASC
-            LIMIT 1
+            UPDATE tasks
+            SET status = 'running'
+            WHERE id = (
+                SELECT t.id
+                FROM tasks t
+                WHERE t.status = 'queued'
+                  AND (t.scheduled_for IS NULL OR t.scheduled_for <= @now)
+                  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'
+                      UNION
+                      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.created_at ASC
+                LIMIT 1
+            )
+            RETURNING id, list_id, title, description, status, scheduled_for,
+                      result, log_path, created_at, started_at, finished_at, commit_type,
+                      model, system_prompt, agent_path
             """;
         cmd.Parameters.AddWithValue("@now", now.ToString("o"));