Prompt before Freebuff takeover

[brandonkachen]

Summary

• Add a CLI-only takeover_prompt state so a second Freebuff instance does not immediately supersede an existing queued or active session.
• Route that state through the waiting-room surface with Take over and Exit choices, including Enter/Esc keyboard handling.
• Keep server session responses distinct from the local CLI prompt state.

Validation

• bun --cwd cli typecheck