feat(devkitd): migrate agent rename to atomic daemon-side rename #410

Part of the devkitd agent migration (#406). Exception to client-execution: runtimeRef.session IS the tmux session name — splitting registry rename from tmux rename-session risks divergence on crash.

Scope

agent rename <old> <new> → daemon-side agent.rename RPC:

• Name-uniqueness check + registry rename under start_mutex
• tmux rename-session / herdr rename executed daemon-side atomically
• Fallback to AgentRegistry.rename on unreachable/unknown-method

Acceptance

• Rename updates registry + runtime session together
• name_in_use coded error on collision
• Console rename pane works via the same path