feat: add WebSocket terminal I/O with server-side DEC 2026 sync

Replace per-keystroke HTTP POST + SSE terminal output with a single
bidirectional WebSocket connection for dramatically lower input latency.
The existing SSE+POST paths remain fully functional as fallback.

Server-side: ws-routes.ts provides /ws/sessions/:id/terminal with 8ms
micro-batching and 16KB flush threshold. Each batch is wrapped in
DEC 2026 synchronized update markers so xterm.js renders atomically —
Ink's DA capability negotiation fails through the PTY→server→WS proxy
chain, so without server-injected markers, cursor-up redraws flicker.

Frontend: _connectWs/_disconnectWs manage per-session WS lifecycle.
Input and resize use WS fast path with HTTP POST fallback. SSE terminal
events are suppressed when WS is active to prevent double rendering.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

package.json

@@ -51,6 +51,7 @@
     "@fastify/compress": "^8.3.1",
     "@fastify/cookie": "^11.0.2",
     "@fastify/static": "^8.0.0",
+    "@fastify/websocket": "^11.2.0",
     "@xterm/addon-fit": "^0.11.0",
     "@xterm/addon-unicode11": "^0.9.0",
     "@xterm/addon-webgl": "^0.19.0",
@@ -76,6 +77,7 @@
     "@types/react": "^19.2.14",
     "@types/uuid": "^10.0.0",
     "@types/web-push": "^3.6.4",
+    "@types/ws": "^8.18.1",
     "@vitest/coverage-v8": "^4.0.18",
     "agent-browser": "^0.6.0",
     "esbuild": "^0.27.3",