feat: implement web terminal for tool instances

- Add TerminalSession backend service for docker exec subprocess management
- Add TerminalManager for WebSocket session lifecycle management
- Create WebSocket endpoint at /ws/tool-instances/{id}/terminal
- Add session cookie authentication and instance ownership verification
- Install xterm.js with fit and web-links addons
- Create TerminalComponent with xterm.js integration
- Create TerminalPage with full-screen terminal view
- Add terminal route at /instances/:id/terminal
- Add terminal button to InstanceList for running instances
- Add terminal and arrow-left icons to icon registry
- Add comprehensive terminal CSS styles (dark theme, responsive)

Quality gates: typecheck ✓, lint ✓, build ✓, Python syntax ✓

apps/web/package-lock.json

@@ -16,7 +16,10 @@
         "react-dom": "^18.2.0",
         "react-router-dom": "^6.20.0",
         "react-simple-code-editor": "^0.14.1",
-        "tailwindcss": "^3.3.0"
+        "tailwindcss": "^3.3.0",
+        "xterm": "^5.3.0",
+        "xterm-addon-fit": "^0.8.0",
+        "xterm-addon-web-links": "^0.9.0"
       },
       "devDependencies": {
         "@testing-library/jest-dom": "^6.9.1",
@@ -6352,6 +6355,33 @@
       "dev": true,
       "license": "MIT"
     },
+    "node_modules/xterm": {
+      "version": "5.3.0",
+      "resolved": "https://registry.npmjs.org/xterm/-/xterm-5.3.0.tgz",
+      "integrity": "sha512-8QqjlekLUFTrU6x7xck1MsPzPA571K5zNqWm0M0oroYEWVOptZ0+ubQSkQ3uxIEhcIHRujJy6emDWX4A7qyFzg==",
+      "deprecated": "This package is now deprecated. Move to @xterm/xterm instead.",
+      "license": "MIT"
+    },
+    "node_modules/xterm-addon-fit": {
+      "version": "0.8.0",
+      "resolved": "https://registry.npmjs.org/xterm-addon-fit/-/xterm-addon-fit-0.8.0.tgz",
+      "integrity": "sha512-yj3Np7XlvxxhYF/EJ7p3KHaMt6OdwQ+HDu573Vx1lRXsVxOcnVJs51RgjZOouIZOczTsskaS+CpXspK81/DLqw==",
+      "deprecated": "This package is now deprecated. Move to @xterm/addon-fit instead.",
+      "license": "MIT",
+      "peerDependencies": {
+        "xterm": "^5.0.0"
+      }
+    },
+    "node_modules/xterm-addon-web-links": {
+      "version": "0.9.0",
+      "resolved": "https://registry.npmjs.org/xterm-addon-web-links/-/xterm-addon-web-links-0.9.0.tgz",
+      "integrity": "sha512-LIzi4jBbPlrKMZF3ihoyqayWyTXAwGfu4yprz1aK2p71e9UKXN6RRzVONR0L+Zd+Ik5tPVI9bwp9e8fDTQh49Q==",
+      "deprecated": "This package is now deprecated. Move to @xterm/addon-web-links instead.",
+      "license": "MIT",
+      "peerDependencies": {
+        "xterm": "^5.0.0"
+      }
+    },
     "node_modules/yallist": {
       "version": "3.1.1",
       "resolved": "https://registry.npmjs.org/yallist/-/yallist-3.1.1.tgz",