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/Dockerfile

@@ -2,7 +2,7 @@ FROM node:20-alpine AS base
 RUN corepack enable && corepack prepare pnpm@latest --activate
 
 FROM base AS deps
-RUN apk add --no-cache libc6-compat
+RUN apk add --no-cache libc6-compat python3 make gcc g++ linux-headers
 WORKDIR /app
 
 # Copy workspace root config + relevant package.jsons for install
@@ -40,7 +40,14 @@ RUN mkdir -p /data/harness && chown nextjs:nodejs /data/harness
 COPY --from=builder /app/apps/harness/public ./apps/harness/public
 COPY --from=builder --chown=nextjs:nodejs /app/apps/harness/.next/standalone ./
 COPY --from=builder --chown=nextjs:nodejs /app/apps/harness/.next/static ./apps/harness/.next/static
+
+# PTY server dependencies + custom server
+COPY --from=builder /app/node_modules/.pnpm/node-pty*/node_modules/node-pty ./node_modules/node-pty
+COPY --from=builder /app/node_modules/.pnpm/ws*/node_modules/ws ./node_modules/ws
+COPY apps/harness/server.js ./server.js
+COPY apps/harness/pty-server.js ./pty-server.js
+
 USER nextjs
 EXPOSE 3100
 ENV PORT=3100
-CMD ["node", "apps/harness/server.js"]
+CMD ["node", "server.js"]