Add contributor attribution to canvas extension cards (#2111)

Show "by @handle" on each canvas extension card and in the details
modal, linking to the contributor's GitHub profile. Author metadata
lives in each extension's canvas.json (and external.json for external
extensions), where the rest of the canvas metadata is stored.

- Store author {name, url} in canvas.json / external.json
- Read author from canvas.json in the website data generator and emit
  it to extensions.json
- Render the GitHub @handle, derived from the profile URL, as the link
  text, with the contributor's name as the link title
- Escape the sanitized author URL before interpolating it into href

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

extensions/diagram-viewer/canvas.json

@@ -3,6 +3,10 @@
   "name": "Diagram Explorer",
   "description": "Render diagrams, click nodes to drill down, and view agent-generated explanations directly in the canvas.",
   "version": "1.0.0",
+  "author": {
+    "name": "Aaron Powell",
+    "url": "https://github.com/aaronpowell"
+  },
   "keywords": [
     "architecture-mapping",
     "canvas-navigation",