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/context-engineering/.github/plugin/plugin.json

@@ -6,5 +6,42 @@
     "name": "Awesome Copilot Community"
   },
   "repository": "https://github.com/github/awesome-copilot",
-  "license": "MIT"
+  "license": "MIT",
+  "tags": [
+    "context",
+    "productivity",
+    "refactoring",
+    "best-practices",
+    "architecture"
+  ],
+  "display": {
+    "ordering": "manual",
+    "show_badge": true
+  },
+  "items": [
+    {
+      "path": "instructions/context-engineering.instructions.md",
+      "kind": "instruction"
+    },
+    {
+      "path": "agents/context-architect.agent.md",
+      "kind": "agent",
+      "usage": "recommended\n\nThe Context Architect agent helps plan multi-file changes by mapping dependencies\nand identifying all relevant files before making modifications.\n\nUse this agent when:\n- Planning refactors that span multiple files\n- Adding features that touch several modules\n- Investigating unfamiliar parts of the codebase\n\nExample usage:\n```\n@context-architect I need to add rate limiting to all API endpoints.\nWhat files are involved and what's the best approach?\n```\n\nFor best results:\n- Describe the high-level goal, not just the immediate task\n- Let the agent search before you provide files\n- Review the context map before approving changes"
+    },
+    {
+      "path": "prompts/context-map.prompt.md",
+      "kind": "prompt",
+      "usage": "optional\n\nUse before any significant change to understand the blast radius.\nProduces a structured map of files, dependencies, and tests."
+    },
+    {
+      "path": "prompts/what-context-needed.prompt.md",
+      "kind": "prompt",
+      "usage": "optional\n\nUse when Copilot gives a generic or incorrect answer.\nAsks Copilot to explicitly list what files it needs to see."
+    },
+    {
+      "path": "prompts/refactor-plan.prompt.md",
+      "kind": "prompt",
+      "usage": "optional\n\nUse for multi-file refactors. Produces a phased plan with\nverification steps and rollback procedures."
+    }
+  ]
 }