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>';
       }
     }
   }

website/src/scripts/pages/plugins.ts

@@ -1,12 +1,12 @@
 /**
- * Collections page functionality
+ * Plugins page functionality
  */
 import { createChoices, getChoicesValues, type Choices } from '../choices';
 import { FuzzySearch, SearchItem } from '../search';
 import { fetchData, debounce, escapeHtml, getGitHubUrl } from '../utils';
 import { setupModal, openFileModal } from '../modal';
 
-interface Collection extends SearchItem {
+interface Plugin extends SearchItem {
   id: string;
   name: string;
   path: string;
@@ -15,16 +15,16 @@ interface Collection extends SearchItem {
   itemCount: number;
 }
 
-interface CollectionsData {
-  items: Collection[];
+interface PluginsData {
+  items: Plugin[];
   filters: {
     tags: string[];
   };
 }
 
-const resourceType = 'collection';
-let allItems: Collection[] = [];
-let search = new FuzzySearch<Collection>();
+const resourceType = 'plugin';
+let allItems: Plugin[] = [];
+let search = new FuzzySearch<Plugin>();
 let tagSelect: Choices;
 let currentFilters = {
   tags: [] as string[],
@@ -49,19 +49,19 @@ function applyFiltersAndRender(): void {
   const activeFilters: string[] = [];
   if (currentFilters.tags.length > 0) activeFilters.push(`${currentFilters.tags.length} tag${currentFilters.tags.length > 1 ? 's' : ''}`);
   if (currentFilters.featured) activeFilters.push('featured');
-  let countText = `${results.length} of ${allItems.length} collections`;
+  let countText = `${results.length} of ${allItems.length} plugins`;
   if (activeFilters.length > 0) {
     countText += ` (filtered by ${activeFilters.join(', ')})`;
   }
   if (countEl) countEl.textContent = countText;
 }
 
-function renderItems(items: Collection[], query = ''): void {
+function renderItems(items: Plugin[], query = ''): void {
   const list = document.getElementById('resource-list');
   if (!list) return;
 
   if (items.length === 0) {
-    list.innerHTML = '<div class="empty-state"><h3>No collections found</h3><p>Try a different search term or adjust filters</p></div>';
+    list.innerHTML = '<div class="empty-state"><h3>No plugins found</h3><p>Try a different search term or adjust filters</p></div>';
     return;
   }
 
@@ -91,13 +91,13 @@ function renderItems(items: Collection[], query = ''): void {
   });
 }
 
-export async function initCollectionsPage(): Promise<void> {
+export async function initPluginsPage(): Promise<void> {
   const list = document.getElementById('resource-list');
   const searchInput = document.getElementById('search-input') as HTMLInputElement;
   const featuredCheckbox = document.getElementById('filter-featured') as HTMLInputElement;
   const clearFiltersBtn = document.getElementById('clear-filters');
 
-  const data = await fetchData<CollectionsData>('collections.json');
+  const data = await fetchData<PluginsData>('plugins.json');
   if (!data || !data.items) {
     if (list) list.innerHTML = '<div class="empty-state"><h3>Failed to load data</h3></div>';
     return;
@@ -105,7 +105,7 @@ export async function initCollectionsPage(): Promise<void> {
 
   allItems = data.items;
 
-  // Map collection items to search items
+  // Map plugin items to search items
   const searchItems = allItems.map(item => ({
     ...item,
     title: item.name,
@@ -140,4 +140,4 @@ export async function initCollectionsPage(): Promise<void> {
 }
 
 // Auto-initialize when DOM is ready
-document.addEventListener('DOMContentLoaded', initCollectionsPage);
+document.addEventListener('DOMContentLoaded', initPluginsPage);