bug(attach): validate target session before writing active pointer

[pinion05]

Summary

agentty attach writes the requested session id as the active pointer without validating that the session exists or is currently running.

Why this is a problem

The command can succeed even when the target session is missing or already exited. The actual failure only appears later on get, text, key, or kill.

Impact

• Misleading CLI UX: attach appears successful when it is not.
• Broken active-session pointers that need later cleanup.
• More confusing failure mode for both humans and agents.

Suggested direction

• Validate that the target session exists and is running during attach.
• Consider applying the same validation policy to explicit --session resolution paths.
• Add tests for attaching nonexistent and exited sessions.

Relevant code

• src/resolveSession.ts#attachSession
• src/resolveSession.ts#resolveTargetSessionId
• src/index.ts#attach