Deprecate Collections in favour of Plugins

Replace Collections with Plugins as first-class citizens in the repo.
With the Copilot CLI v0.409 release making plugins an on-by-default
marketplace, collections are redundant overhead.

## What changed

### Plugin Infrastructure
- Created eng/validate-plugins.mjs (replaces validate-collections.mjs)
- Created eng/create-plugin.mjs (replaces create-collection.mjs)
- Enhanced all 42 plugin.json files with tags, featured, display, and
  items metadata from their corresponding collection.yml files

### Build & Website
- Updated eng/update-readme.mjs to generate plugin docs
- Updated eng/generate-website-data.mjs to emit plugins.json with full
  items array for modal rendering
- Renamed website collections page to plugins (/plugins/)
- Fixed plugin modal to use <div> instead of <pre> for proper styling
- Updated README.md featured section from Collections to Plugins

### Documentation & CI
- Updated CONTRIBUTING.md, AGENTS.md, copilot-instructions.md, PR template
- Updated CI workflows to validate plugins instead of collections
- Replaced docs/README.collections.md with docs/README.plugins.md

### Cleanup
- Removed eng/validate-collections.mjs, eng/create-collection.mjs,
  eng/collection-to-plugin.mjs
- Removed entire collections/ directory (41 .collection.yml + .md files)
- Removed parseCollectionYaml from yaml-parser.mjs
- Removed COLLECTIONS_DIR from constants.mjs

Closes #711

package.json

@@ -11,12 +11,10 @@
     "contributors:report": "node ./eng/contributor-report.mjs",
     "contributors:generate": "all-contributors generate",
     "contributors:check": "all-contributors check",
-    "collection:validate": "node ./eng/validate-collections.mjs",
-    "collection:create": "node ./eng/create-collection.mjs",
+    "plugin:validate": "node ./eng/validate-plugins.mjs",
+    "plugin:create": "node ./eng/create-plugin.mjs",
     "skill:validate": "node ./eng/validate-skills.mjs",
     "skill:create": "node ./eng/create-skill.mjs",
-    "plugin:migrate": "node ./eng/collection-to-plugin.mjs",
-    "plugin:refresh": "PLUGIN_MODE=refresh node ./eng/collection-to-plugin.mjs",
     "website:data": "node ./eng/generate-website-data.mjs",
     "website:dev": "npm run website:data && npm run --prefix website dev",
     "website:build": "npm run build && npm run website:data && npm run --prefix website build",