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/software-engineering-team/.github/plugin/plugin.json

@@ -6,5 +6,50 @@
     "name": "Awesome Copilot Community"
   },
   "repository": "https://github.com/github/awesome-copilot",
-  "license": "MIT"
+  "license": "MIT",
+  "tags": [
+    "team",
+    "enterprise",
+    "security",
+    "devops",
+    "ux",
+    "architecture",
+    "product",
+    "ai-ethics"
+  ],
+  "display": {
+    "ordering": "manual",
+    "show_badge": true
+  },
+  "items": [
+    {
+      "path": "agents/se-ux-ui-designer.agent.md",
+      "kind": "agent",
+      "usage": "## About This Collection\n\nThis collection of 7 agents is based on learnings from [The AI-Native Engineering Flow](https://medium.com/data-science-at-microsoft/the-ai-native-engineering-flow-5de5ffd7d877) experiments at Microsoft, designed to augment software engineering teams across the entire development lifecycle.\n\n**Key Design Principles:**\n- **Standalone**: Each agent works independently without cross-dependencies\n- **Enterprise-ready**: Incorporates OWASP, Zero Trust, WCAG, and Well-Architected frameworks\n- **Lifecycle coverage**: From UX research → Architecture → Development → Security → DevOps\n\n**Agents in this collection:**\n- **SE: UX Designer** - Jobs-to-be-Done analysis and user journey mapping\n- **SE: Tech Writer** - Technical documentation, blogs, ADRs, and user guides\n- **SE: DevOps/CI** - CI/CD debugging and deployment troubleshooting\n- **SE: Product Manager** - GitHub issues with business context and acceptance criteria\n- **SE: Responsible AI** - Bias testing, accessibility (WCAG), and ethical development\n- **SE: Architect** - Architecture reviews with Well-Architected frameworks\n- **SE: Security** - OWASP Top 10, LLM/ML security, and Zero Trust\n\nYou can use individual agents as needed or adopt the full collection for comprehensive team augmentation."
+    },
+    {
+      "path": "agents/se-technical-writer.agent.md",
+      "kind": "agent"
+    },
+    {
+      "path": "agents/se-gitops-ci-specialist.agent.md",
+      "kind": "agent"
+    },
+    {
+      "path": "agents/se-product-manager-advisor.agent.md",
+      "kind": "agent"
+    },
+    {
+      "path": "agents/se-responsible-ai-code.agent.md",
+      "kind": "agent"
+    },
+    {
+      "path": "agents/se-system-architecture-reviewer.agent.md",
+      "kind": "agent"
+    },
+    {
+      "path": "agents/se-security-reviewer.agent.md",
+      "kind": "agent"
+    }
+  ]
 }