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/diff-viewer/manifest.json

@@ -0,0 +1,14 @@
+{
+    "id": "diff-viewer",
+    "name": "Diff Viewer",
+    "version": "1.0.0",
+    "type": "extension",
+    "tier": "browser",
+    "author": "switchboard",
+    "description": "Renders ```diff code blocks with syntax-highlighted additions, deletions, and context lines",
+    "requires": ["chat"],
+    "permissions": [],
+    "tools": [],
+    "surfaces": [],
+    "settings": {}
+}