feat(with-watch): codify immediate startup run contract

[kdy1]

Summary

• document that with-watch runs the delegated command once immediately after watcher setup and baseline capture
• make the runtime contract explicit in the runner and strengthen startup-run regression coverage for passthrough, shell, and exec modes
• align crate docs, project contracts, and public docs around the same startup behavior

Testing

• cargo test
• pnpm --filter public-docs test