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

website/src/scripts/pages/index.ts

@@ -12,12 +12,12 @@ interface Manifest {
     instructions: number;
     skills: number;
     hooks: number;
-    collections: number;
+    plugins: number;
     tools: number;
   };
 }
 
-interface Collection {
+interface Plugin {
   id: string;
   name: string;
   description?: string;
@@ -27,8 +27,8 @@ interface Collection {
   itemCount: number;
 }
 
-interface CollectionsData {
-  items: Collection[];
+interface PluginsData {
+  items: Plugin[];
 }
 
 export async function initHomepage(): Promise<void> {
@@ -36,7 +36,7 @@ export async function initHomepage(): Promise<void> {
   const manifest = await fetchData<Manifest>('manifest.json');
   if (manifest && manifest.counts) {
     // Populate counts in cards
-    const countKeys = ['agents', 'prompts', 'instructions', 'skills', 'hooks', 'collections', 'tools'] as const;
+    const countKeys = ['agents', 'prompts', 'instructions', 'skills', 'hooks', 'plugins', 'tools'] as const;
     countKeys.forEach(key => {
       const countEl = document.querySelector(`.card-count[data-count="${key}"]`);
       if (countEl && manifest.counts[key] !== undefined) {
@@ -97,11 +97,11 @@ export async function initHomepage(): Promise<void> {
     }
   }
 
-  // Load featured collections
-  const collectionsData = await fetchData<CollectionsData>('collections.json');
-  if (collectionsData && collectionsData.items) {
-    const featured = collectionsData.items.filter(c => c.featured).slice(0, 6);
-    const featuredEl = document.getElementById('featured-collections');
+  // Load featured plugins
+  const pluginsData = await fetchData<PluginsData>('plugins.json');
+  if (pluginsData && pluginsData.items) {
+    const featured = pluginsData.items.filter(c => c.featured).slice(0, 6);
+    const featuredEl = document.getElementById('featured-plugins');
     if (featuredEl) {
       if (featured.length > 0) {
         featuredEl.innerHTML = featured.map(c => `
@@ -119,11 +119,11 @@ export async function initHomepage(): Promise<void> {
         featuredEl.querySelectorAll('.card').forEach(el => {
           el.addEventListener('click', () => {
             const path = (el as HTMLElement).dataset.path;
-            if (path) openFileModal(path, 'collection');
+            if (path) openFileModal(path, 'plugin');
           });
         });
       } else {
-        featuredEl.innerHTML = '<p style="text-align: center; color: var(--color-text-muted);">No featured collections yet</p>';
+        featuredEl.innerHTML = '<p style="text-align: center; color: var(--color-text-muted);">No featured plugins yet</p>';
       }
     }
   }