Picker hides when only native plan/build agents exist — plan-first posture loses its escalation affordance

[aarontrowbridge]

Problem — The composer agent picker is gated on hasCustomAgent (any agent with native === false) or the showCustomAgents setting (default off). With Amicode retiring its custom pulse-designer agent (harmoniqs/amicode#389) and defaulting sessions to the read-only plan agent, a server now serves only native agents — so the picker disappears entirely, leaving plan-mode users no visible way to switch to build (only the exit-plan approval or buried settings).

Acceptance Criteria

• The picker is visible whenever more than one selectable (non-subagent, non-hidden) agent exists — including plan/build only
• Single-agent setups keep today's hidden behavior; a lone custom agent still shows the picker