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/ClaudeProcess.cs

@@ -45,6 +45,9 @@ public sealed class ClaudeProcess : IClaudeProcess
         var analyzer = new StreamAnalyzer();
         var lastStderr = new StringBuilder();
 
+        // On cancellation: kill the tree. Killing closes the redirected pipes,
+        // which unblocks the ReadLineAsync loops below (which run without ct
+        // so they reliably drain instead of hanging on cancellation).
         await using var ctr = ct.Register(() =>
         {
             try { process.Kill(entireProcessTree: true); }
@@ -53,26 +56,30 @@ public sealed class ClaudeProcess : IClaudeProcess
 
         var stdoutTask = Task.Run(async () =>
         {
-            while (await process.StandardOutput.ReadLineAsync(ct) is { } line)
+            while (await process.StandardOutput.ReadLineAsync() is { } line)
             {
                 if (string.IsNullOrEmpty(line)) continue;
                 await onStdoutLine(line);
                 analyzer.ProcessLine(line);
             }
-        }, ct);
+        });
 
         var stderrTask = Task.Run(async () =>
         {
-            while (await process.StandardError.ReadLineAsync(ct) is { } line)
+            while (await process.StandardError.ReadLineAsync() is { } line)
             {
                 if (string.IsNullOrEmpty(line)) continue;
                 lastStderr.AppendLine(line);
                 await onStdoutLine($"[stderr] {line}");
             }
-        }, ct);
+        });
 
         await Task.WhenAll(stdoutTask, stderrTask);
-        await process.WaitForExitAsync(ct);
+        await process.WaitForExitAsync(CancellationToken.None);
+
+        // If we were asked to cancel, surface that to the caller now that
+        // the process is fully reaped.
+        ct.ThrowIfCancellationRequested();
 
         var exitCode = process.ExitCode;
         var streamResult = analyzer.GetResult();