fix(#2853): sync .sisyphus state from worktree to main repo before removal

When .sisyphus/ is gitignored, task state written during worktree execution
is lost when the worktree is removed. Fix:

- add worktree-sync.ts: syncSisyphusStateFromWorktree() copies .sisyphus/
  contents from worktree to main repo directory
- update start-work.ts template: documents the sync step as CRITICAL when
  worktree_path is set in boulder.json
- update work-with-pr/SKILL.md: adds explicit sync step before worktree removal
- export from boulder-state index
- test: 5 scenarios covering no-.sisyphus, nested dirs, overwrite stale state

src/features/boulder-state/worktree-sync.ts

@@ -0,0 +1,34 @@
+import { existsSync, cpSync, mkdirSync } from "node:fs"
+import { join } from "node:path"
+import { BOULDER_DIR } from "./constants"
+import { log } from "../../shared/logger"
+
+export function syncSisyphusStateFromWorktree(worktreePath: string, mainRepoPath: string): boolean {
+  const srcDir = join(worktreePath, BOULDER_DIR)
+  const destDir = join(mainRepoPath, BOULDER_DIR)
+
+  if (!existsSync(srcDir)) {
+    log("[worktree-sync] No .sisyphus directory in worktree, nothing to sync", { worktreePath })
+    return true
+  }
+
+  try {
+    if (!existsSync(destDir)) {
+      mkdirSync(destDir, { recursive: true })
+    }
+
+    cpSync(srcDir, destDir, { recursive: true, force: true })
+    log("[worktree-sync] Synced .sisyphus state from worktree to main repo", {
+      worktreePath,
+      mainRepoPath,
+    })
+    return true
+  } catch (err) {
+    log("[worktree-sync] Failed to sync .sisyphus state", {
+      worktreePath,
+      mainRepoPath,
+      error: String(err),
+    })
+    return false
+  }
+}