Implement WebSocket handler and event system

[2witstudios]

Epic 4: WebSocket & Real-Time Push — Issue 1/3

Source: Composite — 3-agent's terminal diff + 2-agent's watcher + 1-agent's handler

Description

WebSocket endpoint with authentication, client tracking, command dispatch, and typed event system.

Acceptance Criteria

• Single endpoint: ws://<host>:<port>/ws?token=<token>
• Token validation from query parameter
• Client set tracking with per-client subscription state
• Command dispatch: ping, terminal:subscribe, terminal:unsubscribe, terminal:input
• Typed discriminated union events: manifest:updated, agent:status, terminal:output, error, pong
• Broadcast helper (send to all connected clients)
• sendEvent helper (send to specific client)
• Proper cleanup on client disconnect
• Tests: connection, auth, command dispatch, event types

Dependencies

• Requires: Create ppg serve command and Fastify server scaffold #63 (server scaffold), Implement token auth system with hashing and rate limiting #64 (auth)

Files to Create/Modify

• src/server/ws/handler.ts (new)
• src/server/ws/events.ts (new)
• src/server/ws/handler.test.ts (new)