Step 6: Gateway — WebSocket server, REST API, session management

## Context

Steps 1-5 build the core agent with persistent memory. Step 6 exposes it over the network so TUI clients, web UIs, and programmatic consumers can interact with it.

## Scope

### HTTP server
- Chi router setup
- `GET /health` — `{"status": "ok", "version": "..."}`
- `GET /ready` — `{"ready": true, "providers": [...]}`
- Graceful shutdown on SIGINT/SIGTERM

### WebSocket protocol
- `GET /ws` — WebSocket upgrade
- Client frames: `hello` (auth), `send` (message), `subscribe`, `cancel`, `session_cmd`
- Server frames: `welcome`, `text_delta`, `tool_progress`, `turn_complete`, `error`, `session_list`, `session_created`, `session_switched`
- Frame types already defined in `internal/types/channel.go`

### REST API
- `POST /api/v1/chat` — non-streaming chat (returns complete response)
- `POST /api/v1/chat/stream` — SSE streaming endpoint
- `GET /api/v1/sessions` — list sessions
- `POST /api/v1/sessions` — create session
- `GET /api/v1/sessions/:id` — get session detail

### Session management
- Create/resume/archive sessions
- Max concurrent sessions (`gateway.max_sessions`)
- Max concurrent turns (`gateway.max_concurrent_turns`)
- Idle session TTL cleanup (`session_idle_ttl_hours`)

### Authentication
- Simple API key auth via `gateway.api_key` config
- `Authorization: Bearer ` header for REST
- `hello` frame with key for WebSocket

### Turn execution bridge
- WebSocket/REST → runtime.Run() bridge
- Stream text deltas back to client in real-time
- Forward tool progress events

## Files to create
```
internal/gateway/
├── server.go # Chi server setup, routes, middleware
├── websocket.go # WebSocket handler + frame protocol
├── rest.go # REST API handlers
├── session_mgr.go # Session lifecycle management
├── auth.go # API key middleware
└── gateway_test.go
```

## Implements
- `yantra start` — starts daemon with gateway
- `yantra serve` — headless API-only mode

## Dependencies
- `github.com/go-chi/chi/v5`
- `nhooyr.io/websocket` (or `gorilla/websocket` already in go.mod)