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/utils.ts

@@ -233,7 +233,7 @@ export function getResourceType(filePath: string): string {
     return "skill";
   if (/(^|\/)hooks\//.test(filePath) && filePath.endsWith("README.md"))
     return "hook";
-  if (filePath.endsWith(".collection.yml")) return "collection";
+  if (filePath.endsWith(".collection.yml")) return "plugin";
   return "unknown";
 }
 
@@ -247,7 +247,7 @@ export function formatResourceType(type: string): string {
     instruction: "📋 Instruction",
     skill: "⚡ Skill",
     hook: "🪝 Hook",
-    collection: "📦 Collection",
+    plugin: "🔌 Plugin",
   };
   return labels[type] || type;
 }
@@ -262,7 +262,7 @@ export function getResourceIcon(type: string): string {
     instruction: "📋",
     skill: "⚡",
     hook: "🪝",
-    collection: "📦",
+    plugin: "🔌",
   };
   return icons[type] || "📄";
 }