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/pages/plugins.astro

@@ -0,0 +1,49 @@
+---
+import BaseLayout from '../layouts/BaseLayout.astro';
+import Modal from '../components/Modal.astro';
+---
+
+<BaseLayout title="Plugins" description="Curated plugins of prompts, agents, and skills for specific workflows" activeNav="plugins">
+  <main id="main-content">
+    <div class="page-header">
+      <div class="container">
+        <h1>🔌 Plugins</h1>
+        <p>Curated plugins of prompts, agents, and skills for specific workflows</p>
+      </div>
+    </div>
+
+    <div class="page-content">
+      <div class="container">
+        <div class="search-bar">
+          <label for="search-input" class="sr-only">Search plugins</label>
+          <input type="text" id="search-input" placeholder="Search plugins..." autocomplete="off">
+        </div>
+        
+        <div class="filters-bar" id="filters-bar">
+          <div class="filter-group">
+            <label for="filter-tag">Tag:</label>
+            <select id="filter-tag" multiple aria-label="Filter by tag"></select>
+          </div>
+          <div class="filter-group">
+            <label class="checkbox-label">
+              <input type="checkbox" id="filter-featured">
+              Featured Only
+            </label>
+          </div>
+          <button id="clear-filters" class="btn btn-secondary btn-small">Clear Filters</button>
+        </div>
+        
+        <div class="results-count" id="results-count" aria-live="polite"></div>
+        <div class="resource-list" id="resource-list" role="list">
+          <div class="loading" aria-live="polite">Loading plugins...</div>
+        </div>
+      </div>
+    </div>
+  </main>
+
+  <Modal />
+
+  <script>
+    import '../scripts/pages/plugins';
+  </script>
+</BaseLayout>