fix(engine): prevent worktree collisions on manual task moves

Two related bugs let two in-progress tasks share a single
.worktrees/<name> directory:

1. The dashboard POST /tasks/:id/move route promoted tasks to
   in-progress without allocating a fresh worktree path, so a queued
   task carrying a stale worktree field from a prior preserveResumeState
   requeue could land in-progress on a directory already held by another
   active task.

2. moveTask({preserveResumeState:true}) kept the worktree pointer on
   requeue. When the on-disk checkout was later removed or reassigned,
   the next dispatch collided with a worktree the scheduler had handed
   to another task.

moveTask now releases the worktree pointer on every reopen-to-todo hop
(branch is kept so committed progress survives via git worktree add
<path> <branch>). A new preserveWorktree option opts internal bounces
out of the release. moveTask also accepts an allocateWorktree callback
that runs under a new cross-task allocation lock in TaskStore, so two
concurrent moves cannot pick the same name from a stale snapshot. Both
the manual-move route and the scheduler dispatch path flow through the
allocator and share the lock.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/engine/src/index.ts

@@ -66,6 +66,7 @@ export {
 } from "./agent-instructions.js";
 export { HEARTBEAT_PROCEDURE, HEARTBEAT_SYSTEM_PROMPT, HEARTBEAT_NO_TASK_SYSTEM_PROMPT } from "./agent-heartbeat.js";
 export { WorktreePool, scanIdleWorktrees, cleanupOrphanedWorktrees, reapOrphanWorktrees } from "./worktree-pool.js";
+export { generateReservedWorktreeName, generateWorktreeName, planTaskWorktreePath, slugify } from "./worktree-names.js";
 export { createLogger, type Logger } from "./logger.js";
 export { isUsageLimitError, UsageLimitPauser } from "./usage-limit-detector.js";
 export { withRateLimitRetry } from "./rate-limit-retry.js";