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

.github/copilot-instructions.md

@@ -55,18 +55,17 @@ The following instructions are only to be applied when performing a code review.
 - [ ] Any bundled assets (scripts, templates, data files) are referenced in the SKILL.md instructions.
 - [ ] Bundled assets are reasonably sized (under 5MB per file).
 
-## Collection file guide
+## Plugin guide
 
-**Only apply to files that end in `.collection.yml`**
+**Only apply to directories in the `plugins/` directory**
 
-- [ ] The collection has a `name` field.
-- [ ] The collection has a `description` field.
+- [ ] The plugin directory contains a `.github/plugin/plugin.json` file.
+- [ ] The plugin directory contains a `README.md` file.
+- [ ] The plugin.json has a `name` field matching the directory name.
+- [ ] The plugin.json has a `description` field.
 - [ ] The `description` field is not empty.
-- [ ] The collection has a `tags` field.
-- [ ] The file name is lower case, with words separated by hyphens.
-- [ ] Each item in the collection has a `path` field.
-- [ ] Each item in the collection has a `kind` field.
-- [ ] The `kind` field value is one of: `prompt`, `instruction`, `agent`, or `skill`.
-- [ ] The collection does not include duplicate items.
-- [ ] The collection does not reference non-existent files.
-- [ ] Each item can have an optional `usage` field describing when to use the item.
+- [ ] The directory name is lower case, with words separated by hyphens.
+- [ ] If `tags` is present, it is an array of lowercase hyphenated strings.
+- [ ] If `items` is present, each item has `path` and `kind` fields.
+- [ ] The `kind` field value is one of: `prompt`, `agent`, `instruction`, `skill`, or `hook`.
+- [ ] The plugin does not reference non-existent files.

.github/pull_request_template.md

@@ -20,9 +20,9 @@
 - [ ] New instruction file.
 - [ ] New prompt file.
 - [ ] New agent file.
-- [ ] New collection file.
+- [ ] New plugin.
 - [ ] New skill file.
-- [ ] Update to existing instruction, prompt, agent, collection or skill.
+- [ ] Update to existing instruction, prompt, agent, plugin, or skill.
 - [ ] Other (please specify):
 
 ---

.github/workflows/deploy-website.yml

@@ -13,7 +13,7 @@ on:
       - "prompts/**"
       - "instructions/**"
       - "skills/**"
-      - "collections/**"
+      - "plugins/**"
       - "cookbook/**"
       - "eng/generate-website-data.mjs"
       - ".github/workflows/deploy-website.yml"

.github/workflows/validate-readme.yml

@@ -7,7 +7,7 @@ on:
       - "instructions/**"
       - "prompts/**"
       - "agents/**"
-      - "collections/**"
+      - "plugins/**"
       - "*.js"
       - "README.md"
       - "docs/**"
@@ -33,8 +33,8 @@ jobs:
       - name: Install dependencies
         run: npm install
 
-      - name: Validate collections
-        run: npm run collection:validate
+      - name: Validate plugins
+        run: npm run plugin:validate
 
       - name: Update README.md
         run: npm start