fix(sandbox): make the daemon URL-transfer routes async

[pedrofrxncx]

Follows #5403/#5406, which converted the daemon's read and rename routes off blocking sync fs calls. write_from_url and upload_to_url in packages/sandbox/daemon/routes/fs.ts still called fs.mkdirSync/fs.rmSync/fs.statSync directly in the request handler.

Per CONTRIBUTING.md rule #1, the sandbox daemon runs on a single-threaded Bun event loop — any sync fs call blocks it from answering its HTTP health probe, and Studio tears down / recovers a sandbox on a single missed probe. mkdirSync/statSync ran on every call to these routes, and rmSync ran on every failed transfer.

The fix swaps each sync call for its already-imported node:fs/promises equivalent (mkdir/rm/stat were already imported and used elsewhere in the file) — a straight 1:1 behavior-preserving substitution, same as the prior read/rename PRs. Renamed the local stat variable to fileStat to avoid shadowing the imported stat function.

To verify: cd packages/sandbox && bunx tsc --noEmit, then bun test packages/sandbox/daemon/routes/fs.test.ts (56 pass, 0 fail — existing coverage of this file passes unchanged since behavior is identical).

Locally ran: bun run fmt, bunx tsc --noEmit (packages/sandbox workspace), and the targeted test file above. Full CI validates the rest.

---

Summary by cubic

Make the sandbox daemon’s URL-transfer routes async to avoid blocking the single-threaded event loop and prevent health probe timeouts. Replaced sync fs calls in write_from_url and upload_to_url with node:fs/promises to keep behavior the same while non-blocking.

• Bug Fixes
  • Swapped mkdirSync/rmSync/statSync for mkdir/rm/stat in packages/sandbox/daemon/routes/fs.ts.
  • Renamed local stat to fileStat to avoid shadowing the imported stat function.

^{Written for commit 10874d9. Summary will update on new commits.}