bug(start): report readiness only after PTY startup and stable session metadata

[pinion05]

Summary

agentty start currently treats socket creation as the success signal, even though PTY startup and final session metadata are completed later.

Why this is a problem

There is a window where start returns success but the PTY is not fully ready yet. During that window, immediate follow-up commands like text or get can still fail. The returned pid can also briefly refer to the worker process rather than the actual PTY process.

Impact

• Flaky command chaining right after start
• Confusing PID semantics for monitoring or cleanup
• Start success that does not yet imply an interactive session is usable

Suggested direction

• Define readiness as PTY spawn + session metadata persistence, not just socket presence.
• Tighten the contract around which PID is returned/stored.
• Add regression coverage for immediate post-start text/get usage.

Relevant code

• src/sessionRuntime.ts#startSession
• src/worker.ts#main
• src/worker.ts#persistRunning