feat(HAI-037): add WorktreePool for recycling idle worktrees

- Add recycleWorktrees setting to control worktree pooling behavior
- Implement WorktreePool class with acquire/release/prepareForTask lifecycle
- Integrate pool into executor: acquire warm worktrees, skip init command for pooled entries
- Integrate pool into merger: release worktrees to pool instead of removing on task completion
- Add comprehensive unit and integration tests for pool, executor, and merger interactions

packages/engine/src/index.ts

@@ -5,3 +5,4 @@ export { Scheduler, type SchedulerOptions } from "./scheduler.js";
 export { aiMergeTask, type MergerOptions } from "./merger.js";
 export { reviewStep, type ReviewType, type ReviewVerdict, type ReviewResult, type ReviewOptions } from "./reviewer.js";
 export { createHaiAgent, type AgentOptions, type AgentResult } from "./pi.js";
+export { WorktreePool } from "./worktree-pool.js";