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

eng/constants.mjs

@@ -57,6 +57,27 @@ Curated collections of related prompts, instructions, and agents organized aroun
 
 Discover our curated collections of prompts, instructions, and agents organized around specific themes and workflows.`,
 
+  pluginsSection: `## 🔌 Plugins
+
+Curated plugins of related prompts, agents, and skills organized around specific themes, workflows, or use cases. Plugins can be installed directly via GitHub Copilot CLI.`,
+
+  pluginsUsage: `### How to Use Plugins
+
+**Browse Plugins:**
+- ⭐ Featured plugins are highlighted and appear at the top of the list
+- Explore themed plugins that group related customizations
+- Each plugin includes prompts, agents, and skills for specific workflows
+- Plugins make it easy to adopt comprehensive toolkits for particular scenarios
+
+**Install Plugins:**
+- Use \\\`copilot plugin install <plugin-name>@awesome-copilot\\\` to install a plugin
+- Or browse to the individual files to copy content manually
+- Plugins help you discover related customizations you might have missed`,
+
+  featuredPluginsSection: `## 🌟 Featured Plugins
+
+Discover our curated plugins of prompts, agents, and skills organized around specific themes and workflows.`,
+
   agentsSection: `## 🤖 Custom Agents
 
 Custom agents for GitHub Copilot, making it easy for users and organizations to "specialize" their Copilot coding agent (CCA) through simple file-based configuration.`,
@@ -151,9 +172,9 @@ const PROMPTS_DIR = path.join(ROOT_FOLDER, "prompts");
 const AGENTS_DIR = path.join(ROOT_FOLDER, "agents");
 const SKILLS_DIR = path.join(ROOT_FOLDER, "skills");
 const HOOKS_DIR = path.join(ROOT_FOLDER, "hooks");
-const COLLECTIONS_DIR = path.join(ROOT_FOLDER, "collections");
+const PLUGINS_DIR = path.join(ROOT_FOLDER, "plugins");
 const COOKBOOK_DIR = path.join(ROOT_FOLDER, "cookbook");
-const MAX_COLLECTION_ITEMS = 50;
+const MAX_PLUGIN_ITEMS = 50;
 
 // Agent Skills validation constants
 const SKILL_NAME_MIN_LENGTH = 1;
@@ -166,12 +187,12 @@ const DOCS_DIR = path.join(ROOT_FOLDER, "docs");
 export {
   AGENTS_DIR,
   AKA_INSTALL_URLS,
-  COLLECTIONS_DIR,
+  PLUGINS_DIR,
   COOKBOOK_DIR,
   DOCS_DIR,
   HOOKS_DIR,
   INSTRUCTIONS_DIR,
-  MAX_COLLECTION_ITEMS,
+  MAX_PLUGIN_ITEMS,
   PROMPTS_DIR,
   repoBaseUrl,
   ROOT_FOLDER,