Add Site Studio canvas extension (#2117)

* Add Site Studio canvas extension

Site Studio is a canvas extension for planning, drafting, and tracking a
personal website section by section. It gives you and your agent a shared
dashboard with a status board, an autosaving content editor (with AI-draft
and [Sample: ...] placeholders), and a live feed of every change and milestone.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Address Copilot review feedback

Robustness, prototype-pollution safety, and accessibility fixes from the
Copilot code review on #2117:

- Add "ai_request" to VALID_CHANGE_TYPES so log_change accepts the change
  type the server itself emits (e.g. /api/request-generation).
- Bound the git branch lookup with timeout + maxBuffer so a hung git can't
  block the extension process and canvas UI.
- Serialize state persistence via a promise queue and snapshot state
  synchronously, so concurrent mutations can't clobber newer state on disk.
- Enforce a maximum request body size in readBodyJson to avoid unbounded
  memory use on the loopback server.
- Guard the /events handler against a missing/closed instance instead of
  throwing when servers.get(instanceId) is undefined.
- Reject unsafe field names (__proto__, prototype, constructor) in
  upsertSectionContent and use an own-property check in deleteSectionContent
  to prevent prototype pollution.
- Make the "Generate with AI" info tooltip reachable by keyboard and screen
  readers (focusable, labelled) instead of mouse-hover only.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: ayangupt <ayangupt@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

extensions/site-studio/canvas.json

@@ -0,0 +1,24 @@
+{
+  "id": "site-studio",
+  "name": "Site Studio",
+  "description": "Plan, draft, and track a personal website section by section — a shared canvas where you and your agent author content, watch progress, and review every change.",
+  "version": "1.0.0",
+  "keywords": [
+    "agent-collaboration",
+    "content-authoring",
+    "personal-website",
+    "progress-tracking",
+    "section-planning",
+    "site-builder"
+  ],
+  "screenshots": {
+    "icon": {
+      "path": "assets/preview.png",
+      "type": "image/png"
+    },
+    "gallery": {
+      "path": "assets/preview.png",
+      "type": "image/png"
+    }
+  }
+}

extensions/site-studio/package.json

@@ -0,0 +1,18 @@
+{
+  "name": "site-studio",
+  "version": "1.0.0",
+  "type": "module",
+  "main": "extension.mjs",
+  "dependencies": {
+    "@github/copilot-sdk": "latest"
+  },
+  "description": "A shared canvas for planning, drafting, and tracking a personal website section by section, collaboratively with your agent.",
+  "keywords": [
+    "agent-collaboration",
+    "content-authoring",
+    "personal-website",
+    "progress-tracking",
+    "section-planning",
+    "site-builder"
+  ]
+}