feat(worker): batch MCP tools for the external endpoint

Add seven best-effort batch variants of the single-entity external MCP
tools: batch_get_tasks, batch_add_tasks, batch_update_task_status,
batch_cancel_tasks, batch_delete_tasks, batch_set_my_day, and
batch_cleanup_task_worktrees. Each loops the existing ExternalMcpService
methods sequentially (scoped DbContext is not thread-safe), returns a
per-item result array so a failing item never aborts the rest, and
rejects empty or over-100-item batches. Merge/review stay single-task.

src/ClaudeDo.Worker/CLAUDE.md

@@ -34,6 +34,7 @@ Interfaces (e.g. `IQueueWaker`, `IPrimeClock`, `ITaskStateService`) live in an `
 - **StaleTaskRecovery** — startup-only service; calls `TaskStateService.RecoverStaleRunningAsync` to flip orphaned `Running` rows to `Failed`.
 - **External/*** — always-on MCP tools for general Claude sessions, scoped to *starting* and *observing* sessions (no worktree/merge, multi-turn, planning, or app-settings writes). Auth via optional `X-ClaudeDo-Key` header. Registered explicitly in `Program.cs`'s external app via `.WithTools<T>()`. Organized by concern:
   - `ExternalMcpService` — task CRUD + execution: `ListTaskLists`, `ListTasks`, `GetTask`, `AddTask`, `AddSubtask`, `UpdateTask`, `UpdateTaskStatus` (`Idle` / `Queued`), `GetTaskStatusValues`, `ReviewTask` (`approve` / `reject_rerun` / `reject_park` / `cancel` for a WaitingForReview task), `RunTaskNow`, `ContinueTask`, `CancelTask`, `DeleteTask`; worktree/git: `GetTaskWorktree`, `GetTaskDiff`, `MergeTask`, `ListWorktrees`, `CleanupTaskWorktree`
+  - `BatchMcpTools` — best-effort batch variants that loop the `ExternalMcpService` single-entity methods (sequential — the scoped DbContext is not thread-safe; merge/review stay single-task): `BatchGetTasks`, `BatchAddTasks`, `BatchUpdateTaskStatus`, `BatchCancelTasks`, `BatchDeleteTasks`, `BatchSetMyDay`, `BatchCleanupTaskWorktrees`. Every tool returns a per-item result array ({ id/index, ok, error?, … }) — a failing item never aborts the rest — and rejects batches over 100 items.
   - `ListMcpTools` — `CreateList`, `UpdateList`, `DeleteList`
   - `ConfigMcpTools` — `GetListConfig`, `SetListConfig`, `GetTaskConfig`, `SetTaskConfig`
   - `RunHistoryMcpTools` — `ListRuns`, `GetRun`, `GetTaskLog` (latest run's log, tail-capped at 256 KB)