Feat v0.2.9 builtin extension retirement

Stop auto-seeding 6 chat-centric extensions on startup. Move them to
standard packages with js/ layout and "requires": ["chat"] metadata.
Delete SeedBuiltinPackages, seed tests, Dockerfile COPY, and
extensions/builtin/ directory.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/js-sandbox/manifest.json

@@ -0,0 +1,30 @@
+{
+    "id": "js-sandbox",
+    "name": "JavaScript Sandbox",
+    "version": "1.0.0",
+    "type": "extension",
+    "tier": "browser",
+    "author": "switchboard",
+    "description": "Executes JavaScript in a sandboxed iframe. Allows the LLM to run code, verify calculations, and transform data — no server compute required.",
+    "requires": ["chat"],
+    "permissions": [],
+    "tools": [
+        {
+            "name": "js_eval",
+            "display_name": "JavaScript Eval",
+            "description": "Execute JavaScript code in a secure browser sandbox. Use this to: verify calculations, run algorithms, transform data, test code snippets, parse/format strings, or any computation. Console output (log/warn/error) is captured. The last expression's value is returned as the result. Has no network access, no DOM access, no access to the parent page. Timeout: 10 seconds.",
+            "parameters": {
+                "type": "object",
+                "properties": {
+                    "code": {
+                        "type": "string",
+                        "description": "JavaScript code to execute. Use console.log() for output. The value of the last expression is returned automatically."
+                    }
+                },
+                "required": ["code"]
+            }
+        }
+    ],
+    "surfaces": [],
+    "settings": {}
+}