Canvas manifest implementation for extensions (#2029)

* Add keyword display to extension cards on website

- Add .resource-keywords and .keyword-tag CSS styles for rendering keyword badges
- Update renderExtensionsHtml() to display keywords below extension description
- Keywords now visible on the website extensions page with styled badges
- Regenerate website data to include keyword metadata

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

* Canvas manifest implementation for all extensions

Add per-extension canvas manifests with:
- Structured canvas metadata (name, description, version, keywords)
- Screenshot definitions (icon and gallery with path/type)
- Relative paths for images within each extension directory

Enhance extension metadata:
- Generate meaningful descriptions from source analysis
- Extract and assign keywords for discoverability
- Store metadata in package.json and extension source files

Update website rendering and data generation:
- Include keywords in extension cards and search index
- Add per-extension canvas.json files for independent evolution
- Support screenshot metadata in manifest structure
- Generate extensions.json with full canonical paths for website

All 9 local canvas extensions now have complete manifests with descriptions, keywords, and screenshot references.

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

* Tweaking some descriptions

* Fix description priority to prefer package.json over in-source metadata

Reverse the priority in canvasDescription so that package.json descriptions
(which contain the enhanced, manually-curated descriptions) take precedence
over older in-source descriptions extracted from createCanvas(...) calls.

This prevents regression when npm run website:data regenerates outputs,
ensuring that committed canvas.json files maintain the current descriptions.

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

* Fix canvas validation to skip external.json file

The validation script was treating extensions/external.json as if it were
a directory, causing false validation failures. Added check to skip files
(identified by presence of dot in filename) and only validate actual
canvas extension directories.

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

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

extensions/release-notes-showcase/package.json

@@ -3,8 +3,16 @@
   "version": "1.0.0",
   "type": "module",
   "main": "extension.mjs",
-  "description": "Release notes canvas for building polished release communications and exports.",
   "dependencies": {
     "@github/copilot-sdk": "1.0.1"
-  }
+  },
+  "description": "Compose and refine launch-ready release notes with contributor callouts and export-friendly output.",
+  "keywords": [
+    "release-notes",
+    "launch-summary",
+    "changelog",
+    "contributor-callouts",
+    "product-updates",
+    "email-export"
+  ]
 }