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

plugins/mcp-m365-copilot/.github/plugin/plugin.json

@@ -6,5 +6,39 @@
     "name": "Awesome Copilot Community"
   },
   "repository": "https://github.com/github/awesome-copilot",
-  "license": "MIT"
+  "license": "MIT",
+  "tags": [
+    "mcp",
+    "m365-copilot",
+    "declarative-agents",
+    "api-plugins",
+    "model-context-protocol",
+    "adaptive-cards"
+  ],
+  "display": {
+    "show_badge": true
+  },
+  "items": [
+    {
+      "path": "prompts/mcp-create-declarative-agent.prompt.md",
+      "kind": "prompt"
+    },
+    {
+      "path": "prompts/mcp-create-adaptive-cards.prompt.md",
+      "kind": "prompt"
+    },
+    {
+      "path": "prompts/mcp-deploy-manage-agents.prompt.md",
+      "kind": "prompt"
+    },
+    {
+      "path": "instructions/mcp-m365-copilot.instructions.md",
+      "kind": "instruction"
+    },
+    {
+      "path": "agents/mcp-m365-agent-expert.agent.md",
+      "kind": "agent",
+      "usage": "recommended\n\nThis chat mode provides expert guidance for building MCP-based declarative agents for Microsoft 365 Copilot.\n\nThis chat mode is ideal for:\n- Creating new declarative agents with MCP integration\n- Designing Adaptive Cards for visual responses\n- Configuring OAuth 2.0 or SSO authentication\n- Setting up response semantics and data extraction\n- Troubleshooting deployment and governance issues\n- Learning MCP best practices for M365 Copilot\n\nTo get the best results, consider:\n- Using the instruction file to set context for all Copilot interactions\n- Using prompts to generate initial agent structure and configurations\n- Switching to the expert chat mode for detailed implementation help\n- Providing specific details about your MCP server, tools, and business scenario"
+    }
+  ]
 }