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/plugins.ts

@@ -0,0 +1,143 @@
+/**
+ * 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 Plugin extends SearchItem {
+  id: string;
+  name: string;
+  path: string;
+  tags?: string[];
+  featured?: boolean;
+  itemCount: number;
+}
+
+interface PluginsData {
+  items: Plugin[];
+  filters: {
+    tags: string[];
+  };
+}
+
+const resourceType = 'plugin';
+let allItems: Plugin[] = [];
+let search = new FuzzySearch<Plugin>();
+let tagSelect: Choices;
+let currentFilters = {
+  tags: [] as string[],
+  featured: false
+};
+
+function applyFiltersAndRender(): void {
+  const searchInput = document.getElementById('search-input') as HTMLInputElement;
+  const countEl = document.getElementById('results-count');
+  const query = searchInput?.value || '';
+
+  let results = query ? search.search(query) : [...allItems];
+
+  if (currentFilters.tags.length > 0) {
+    results = results.filter(item => item.tags?.some(tag => currentFilters.tags.includes(tag)));
+  }
+  if (currentFilters.featured) {
+    results = results.filter(item => item.featured);
+  }
+
+  renderItems(results, query);
+  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} plugins`;
+  if (activeFilters.length > 0) {
+    countText += ` (filtered by ${activeFilters.join(', ')})`;
+  }
+  if (countEl) countEl.textContent = countText;
+}
+
+function renderItems(items: Plugin[], query = ''): void {
+  const list = document.getElementById('resource-list');
+  if (!list) return;
+
+  if (items.length === 0) {
+    list.innerHTML = '<div class="empty-state"><h3>No plugins found</h3><p>Try a different search term or adjust filters</p></div>';
+    return;
+  }
+
+  list.innerHTML = items.map(item => `
+    <div class="resource-item" data-path="${escapeHtml(item.path)}">
+      <div class="resource-info">
+        <div class="resource-title">${item.featured ? '⭐ ' : ''}${query ? search.highlight(item.name, query) : escapeHtml(item.name)}</div>
+        <div class="resource-description">${escapeHtml(item.description || 'No description')}</div>
+        <div class="resource-meta">
+          <span class="resource-tag">${item.itemCount} items</span>
+          ${item.tags?.slice(0, 4).map(t => `<span class="resource-tag">${escapeHtml(t)}</span>`).join('') || ''}
+          ${item.tags && item.tags.length > 4 ? `<span class="resource-tag">+${item.tags.length - 4} more</span>` : ''}
+        </div>
+      </div>
+      <div class="resource-actions">
+        <a href="${getGitHubUrl(item.path)}" class="btn btn-secondary" target="_blank" onclick="event.stopPropagation()" title="View on GitHub">GitHub</a>
+      </div>
+    </div>
+  `).join('');
+
+  // Add click handlers
+  list.querySelectorAll('.resource-item').forEach(el => {
+    el.addEventListener('click', () => {
+      const path = (el as HTMLElement).dataset.path;
+      if (path) openFileModal(path, resourceType);
+    });
+  });
+}
+
+export async function initPluginsPage(): Promise<void> {
+  const list = document.getElementById('resource-list');
+  const searchInput = document.getElementById('search-input') as HTMLInputElement;
+  const featuredCheckbox = document.getElementById('filter-featured') as HTMLInputElement;
+  const clearFiltersBtn = document.getElementById('clear-filters');
+
+  const data = await fetchData<PluginsData>('plugins.json');
+  if (!data || !data.items) {
+    if (list) list.innerHTML = '<div class="empty-state"><h3>Failed to load data</h3></div>';
+    return;
+  }
+
+  allItems = data.items;
+
+  // Map plugin items to search items
+  const searchItems = allItems.map(item => ({
+    ...item,
+    title: item.name,
+    searchText: `${item.name} ${item.description} ${item.tags?.join(' ') || ''}`.toLowerCase()
+  }));
+  search.setItems(searchItems);
+
+  tagSelect = createChoices('#filter-tag', { placeholderValue: 'All Tags' });
+  tagSelect.setChoices(data.filters.tags.map(t => ({ value: t, label: t })), 'value', 'label', true);
+  document.getElementById('filter-tag')?.addEventListener('change', () => {
+    currentFilters.tags = getChoicesValues(tagSelect);
+    applyFiltersAndRender();
+  });
+
+  applyFiltersAndRender();
+  searchInput?.addEventListener('input', debounce(() => applyFiltersAndRender(), 200));
+
+  featuredCheckbox?.addEventListener('change', () => {
+    currentFilters.featured = featuredCheckbox.checked;
+    applyFiltersAndRender();
+  });
+
+  clearFiltersBtn?.addEventListener('click', () => {
+    currentFilters = { tags: [], featured: false };
+    tagSelect.removeActiveItems();
+    if (featuredCheckbox) featuredCheckbox.checked = false;
+    if (searchInput) searchInput.value = '';
+    applyFiltersAndRender();
+  });
+
+  setupModal();
+}
+
+// Auto-initialize when DOM is ready
+document.addEventListener('DOMContentLoaded', initPluginsPage);