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/diagram-viewer/canvas.json

@@ -0,0 +1,24 @@
+{
+  "id": "diagram",
+  "name": "Diagram Explorer",
+  "description": "Render diagrams, click nodes to drill down, and view agent-generated explanations directly in the canvas.",
+  "version": "1.0.0",
+  "keywords": [
+    "architecture-mapping",
+    "canvas-navigation",
+    "exploratory-analysis",
+    "interactive-diagrams",
+    "node-drilldown",
+    "relationship-visualization"
+  ],
+  "screenshots": {
+    "icon": {
+      "path": "assets/preview.png",
+      "type": "image/png"
+    },
+    "gallery": {
+      "path": "assets/preview.png",
+      "type": "image/png"
+    }
+  }
+}