docs(workflows): add file_sync_tests workflow and enforce tests in deploy flow ...

- file_sync_tests.md: all-in-one commands, env var reference, test case table,
  and troubleshooting guide for the 8 file sync integration tests
- deploy_to_production.md: added mandatory pre-deploy gate (run SmallFileSync
  cases 1-4 before any commit to sync-related files) and mandatory post-deploy
  re-run of the same tests with rollback instruction if they fail