feat: add session model switching for plugin-driven model routing

Adds first-class session model switching that allows plugins to dynamically
change the active model for a session. Model changes persist across turns,
update the UI, and are inherited by subagents.

Key changes:
- Add model/modelVariant fields to Session.Info with database migration
- Add Session.setModel() backend API for programmatic model changes
- Add PATCH /session/:sessionID/model HTTP route
- Chat.message hook mutations flow through Session.setModel()
- TUI syncs session model via reactive effect on sync.session.get()

Author: John Ferguson

packages/opencode/migration/20260401004113_add_session_model/migration.sql

@@ -0,0 +1,2 @@
+ALTER TABLE `session` ADD `model` text;--> statement-breakpoint
+ALTER TABLE `session` ADD `model_variant` text;