Add interactive PTY Chat tab with xterm.js terminal emulator

Browser-based interactive terminal sessions with agent CLIs via
WebSocket + node-pty. Supports full TUI rendering (colors, cursor,
ctrl-c) through xterm.js in the browser.

Architecture: xterm.js ←WebSocket→ pty-server.js ←PTY→ agent CLI

- Extract shared buildAgentEnv() from executor into agent-env.ts
- Add internal /api/agents/[id]/env endpoint for PTY server
- Add pty-server.js (WebSocket + node-pty, max 3 sessions, 2hr cleanup)
- Add custom server.js wrapping Next.js with WebSocket upgrade
- Add ChatTab component with agent selector and terminal
- Wire CHAT tab into dashboard nav and render
- Configure serverExternalPackages for node-pty
- Update Dockerfile with build tools and custom server
- Bump k8s memory limit 1Gi → 2Gi for PTY sessions

apps/harness/next.config.js

@@ -2,6 +2,7 @@
 const nextConfig = {
   output: "standalone",
   instrumentationHook: true,
+  serverExternalPackages: ["node-pty"],
 };
 
 module.exports = nextConfig;