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/yaml-parser.mjs

@@ -1,4 +1,4 @@
-// YAML parser for collection files and frontmatter parsing using vfile-matter
+// YAML parser for frontmatter parsing using vfile-matter
 import fs from "fs";
 import yaml from "js-yaml";
 import path from "path";
@@ -14,25 +14,6 @@ function safeFileOperation(operation, filePath, defaultValue = null) {
   }
 }
 
-/**
- * Parse a collection YAML file (.collection.yml)
- * Collections are pure YAML files without frontmatter delimiters
- * @param {string} filePath - Path to the collection file
- * @returns {object|null} Parsed collection object or null on error
- */
-function parseCollectionYaml(filePath) {
-  return safeFileOperation(
-    () => {
-      const content = fs.readFileSync(filePath, "utf8");
-
-      // Collections are pure YAML files, parse directly with js-yaml
-      return yaml.load(content, { schema: yaml.JSON_SCHEMA });
-    },
-    filePath,
-    null
-  );
-}
-
 /**
  * Parse frontmatter from a markdown file using vfile-matter
  * Works with any markdown file that has YAML frontmatter (agents, prompts, instructions)
@@ -292,7 +273,6 @@ export {
   extractAgentMetadata,
   extractMcpServerConfigs,
   extractMcpServers,
-  parseCollectionYaml,
   parseFrontmatter,
   parseSkillMetadata,
   parseHookMetadata,