Create ppg serve command and Fastify server scaffold

[2witstudios]

Epic 2: Server Foundation — Issue 1/5

Source: Composite — 2-agent's lifecycle + serve-auth's security

Description

Create the ppg serve command and Fastify server scaffold with plugin architecture, CORS, health endpoint, graceful shutdown, and state file management.

Acceptance Criteria

• src/commands/serve.ts — Commander.js registration with --port, --host, --token, --daemon, --json
• src/server/index.ts — Fastify instance with CORS, plugin registration hooks
• Health endpoint: GET /health returns { status: "ok", uptime, version }
• Graceful shutdown on SIGTERM/SIGINT
• State file (serve.json) with 0o600 permissions
• Path helpers: serveConfigPath(), servePidPath() in lib/paths.ts
• QR code display on startup (token + connection URL)
• Network interface detection for LAN IP

Files to Create/Modify

• src/commands/serve.ts (new)
• src/server/index.ts (new)
• src/lib/paths.ts (add serve paths)