fix(sandbox): make publish()'s untracked-dir expansion async

[pedrofrxncx]

Continues the daemon's sync-fs → async-fs hardening pass (#5471 discard, #5481 decofile-block validation, #5497 hooks-dir mkdtemp) for the same reason: any blocking *Sync fs call on the daemon's single-threaded Bun event loop stalls the /health probe Studio polls, which can get a live sandbox marked dead.

Bug: publish()'s expandUntrackedDirs() helper called fs.statSync from inside the already-async publish() handler. git status --porcelain collapses an untracked directory into one ?? dir/ entry, so this runs on every publish that touches a fresh untracked directory — e.g. the very common case of a brand-new .deco/blocks/ dir before its first commit — blocking the event loop for the syscall's duration.

Fix: swap fs.statSync (node:fs) for fs.promises.stat, awaited at the one call site inside publish(). Behavior-preserving: same directory-vs-file detection, same fallback to false on a stat error (e.g. a raced deletion), same single call site. Net diff: +9/-3.

Verify: cd packages/sandbox && bunx tsc --noEmit (clean) and bun test packages/sandbox/daemon/routes/git.test.ts (51 pass) — several existing publish() tests (e.g. "publish() commits a valid decofile block") write directly into a fresh untracked .deco/blocks/ dir, exercising this exact code path.

Locally ran: bun run fmt, targeted tsc --noEmit in packages/sandbox, the single test file above, and bunx oxlint packages/sandbox/daemon/routes/git.ts (0 warnings/errors). Full CI validates the rest.

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Summary by cubic

Make publish()’s untracked-dir expansion async to avoid blocking the daemon’s event loop and stalling the /health probe. Replaces fs.statSync with fs.promises.stat to prevent stalls when publishing into fresh untracked dirs (e.g. .deco/blocks/).

• Bug Fixes
  • Converted expandUntrackedDirs to async and awaited it in publish().
  • Preserved behavior (dir vs file check, fallback on stat errors).
  • Removes event-loop stalls on publishes touching untracked directories.

^{Written for commit db9ca1d. Summary will update on new commits.}