refactor(run): migrate watch mode to run sessions

[GaspardKirira]

Summary

Replace the current watch loops with the structured run-session lifecycle.

Requirements

• Use engine-provided watch inputs.
• Batch filesystem changes.
• Request affected target rebuilds.
• Keep the current process during failed builds.
• Restart after successful builds.
• Emit structured lifecycle events.
• Remove direct spinner calls from watch logic.

Acceptance criteria

• Script watch mode uses run sessions.
• Project watch mode uses run sessions.
• Successful rebuilds restart the application.
• Failed rebuilds preserve the current process.
• Watch logic does not render terminal output.