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-lock.json

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@@ -2118,6 +2167,16 @@
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