- Add `--model <modelId>` and `--provider <provider>` flags to `mosaic tui`
- Pass initial model/provider selection through to TuiApp as props
- Fetch available models from gateway `/api/providers/models` on startup
- Show active model/provider in TUI header status bar
- Implement `/model`, `/provider`, and `/help` slash commands for mid-session switching
- `/model` with no args lists available models; `/model <id>` or `/model <p>/<id>` switches
- `/provider` with no args lists providers; `/provider <name>` switches and auto-selects first model
- Send `provider` and `modelId` fields in every WebSocket `message` emit
- Extract gateway REST client to `packages/cli/src/tui/gateway-api.ts`
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
