bug(kill): do not report success before the session has actually exited

[pinion05]

Summary

agentty kill can return success before the PTY has actually exited.

Why this is a problem

killSession() waits up to a fixed timeout and then returns without verifying that the session truly transitioned to exited.

Impact

• Users can believe a session was cleaned up while the PTY is still alive.
• Active session pointers may be cleared even though the underlying process is still running.
• This can leave orphaned processes and misleading runtime state.

Suggested direction

• Treat kill timeout as a failure instead of silent success.
• Verify final process/session state before returning success.
• Add a regression test with a slow-to-exit or TERM-resistant process.

Relevant code

• src/sessionRuntime.ts#waitForExited
• src/sessionRuntime.ts#killSession
• src/worker.ts#requestKill