feat: add direct PTY grid with expand/select mode for text copying

Replace tmux-based grid with direct PTY rendering. Add [MAX] button
to expand a pane to fullscreen and [SEL] to enable native text
selection within the expanded pane. Esc exits select/expanded mode.

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

package.json

@@ -29,6 +29,7 @@
     "typescript": "^5"
   },
   "dependencies": {
-    "@opentui/core": "^0.1.81"
+    "@opentui/core": "^0.1.81",
+    "node-pty": "^1.1.0"
   }
 }