Profile switch: emit WS event so client updates UI immediately ...

ProfileSwitched event emitted by switch_profile tool via current_event_sink.
Client handles profile_switched: updates chat header, profile selector,
and local sessions[] — no page refresh needed.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Add switch_profile tool for automatic profile switching ...

Navi can now switch her own profile mid-session when the task domain
changes. The new profile (tools + system prompt) takes effect from the
next user message. Injected with session_store + profile_registry like
SpawnAgentTool. Added to all profiles' enabled_tools and persona.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>