feat(devkitd): migrate agent open to daemon lookup + client-side focus #409

Part of the devkitd agent migration (#406). Model: daemon returns location data; client executes GUI focus.

Scope

agent open <name> → daemon path:

• agent.lookup → runtimeRef/pid; client does findTerminal/focusTerminal (osascript/iTerm) — GUI focus stays client-side
• herdr focus via existing herdr API path
• Fallback to focusAgent on unreachable/unknown-method

Acceptance

• agent open focuses the correct terminal for daemon-registered agents
• Legacy-registered agents still resolve