fix(sandbox): supervise preview source sync

[iamjr15]

Why

A transient Daytona FUSE read denial could terminate the durable-to-native source synchronizer while the Vite child stayed healthy. Process reuse then saw a live port and returned a permanently stale preview.

What changed

• Retry transient durable-source read failures immediately and within a bounded grace period.
• Treat the app and synchronizer as one managed process unit; a dead synchronizer terminates the app so the existing bounded supervisor restarts both.
• Extend immutable snapshot smoke checks to cover transient recovery, live source propagation, and sync-child failure.
• Document the managed-preview liveness contract at the sandbox and agent-worker boundaries.

Verification

• pnpm lint
• pnpm typecheck
• pnpm turbo build --force
• pnpm deadcode
• pnpm architecture:check
• pnpm turbo skills:build
• Local lifecycle smoke: simulated the observed EPERM, verified source recovery, verified live update propagation, killed the sync child, and confirmed the preview parent exited non-zero.

All repository commands ran with Node 24.18.0 and pnpm 11.15.0. Production browser QA follows immutable snapshot publication and promotion.