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/create-plugin.mjs

@@ -0,0 +1,191 @@
+#!/usr/bin/env node
+
+import fs from "fs";
+import path from "path";
+import readline from "readline";
+import { ROOT_FOLDER } from "./constants.mjs";
+
+const PLUGINS_DIR = path.join(ROOT_FOLDER, "plugins");
+
+const rl = readline.createInterface({
+  input: process.stdin,
+  output: process.stdout,
+});
+
+function prompt(question) {
+  return new Promise((resolve) => {
+    rl.question(question, resolve);
+  });
+}
+
+function parseArgs() {
+  const args = process.argv.slice(2);
+  const out = { name: undefined, tags: undefined };
+
+  for (let i = 0; i < args.length; i++) {
+    const a = args[i];
+    if (a === "--name" || a === "-n") {
+      out.name = args[i + 1];
+      i++;
+    } else if (a.startsWith("--name=")) {
+      out.name = a.split("=")[1];
+    } else if (a === "--tags" || a === "-t") {
+      out.tags = args[i + 1];
+      i++;
+    } else if (a.startsWith("--tags=")) {
+      out.tags = a.split("=")[1];
+    } else if (!a.startsWith("-") && !out.name) {
+      // first positional -> name
+      out.name = a;
+    } else if (!a.startsWith("-") && out.name && !out.tags) {
+      // second positional -> tags
+      out.tags = a;
+    }
+  }
+
+  if (Array.isArray(out.tags)) {
+    out.tags = out.tags.join(",");
+  }
+
+  return out;
+}
+
+async function createPlugin() {
+  try {
+    console.log("🔌 Plugin Creator");
+    console.log("This tool will help you create a new plugin.\n");
+
+    const parsed = parseArgs();
+
+    // Get plugin ID
+    let pluginId = parsed.name;
+    if (!pluginId) {
+      pluginId = await prompt("Plugin ID (lowercase, hyphens only): ");
+    }
+
+    if (!pluginId) {
+      console.error("❌ Plugin ID is required");
+      process.exit(1);
+    }
+
+    if (!/^[a-z0-9-]+$/.test(pluginId)) {
+      console.error(
+        "❌ Plugin ID must contain only lowercase letters, numbers, and hyphens"
+      );
+      process.exit(1);
+    }
+
+    const pluginDir = path.join(PLUGINS_DIR, pluginId);
+
+    // Check if plugin already exists
+    if (fs.existsSync(pluginDir)) {
+      console.log(
+        `⚠️  Plugin ${pluginId} already exists at ${pluginDir}`
+      );
+      console.log("💡 Please edit that plugin instead or choose a different ID.");
+      process.exit(1);
+    }
+
+    // Get display name
+    const defaultDisplayName = pluginId
+      .split("-")
+      .map((word) => word.charAt(0).toUpperCase() + word.slice(1))
+      .join(" ");
+
+    let displayName = await prompt(
+      `Display name (default: ${defaultDisplayName}): `
+    );
+    if (!displayName.trim()) {
+      displayName = defaultDisplayName;
+    }
+
+    // Get description
+    const defaultDescription = `A plugin for ${displayName.toLowerCase()}.`;
+    let description = await prompt(
+      `Description (default: ${defaultDescription}): `
+    );
+    if (!description.trim()) {
+      description = defaultDescription;
+    }
+
+    // Get tags
+    let tags = [];
+    let tagInput = parsed.tags;
+    if (!tagInput) {
+      tagInput = await prompt(
+        "Tags (comma-separated, or press Enter for defaults): "
+      );
+    }
+
+    if (tagInput && tagInput.toString().trim()) {
+      tags = tagInput
+        .toString()
+        .split(",")
+        .map((tag) => tag.trim())
+        .filter((tag) => tag);
+    } else {
+      tags = pluginId.split("-").slice(0, 3);
+    }
+
+    // Create directory structure
+    const githubPluginDir = path.join(pluginDir, ".github", "plugin");
+    fs.mkdirSync(githubPluginDir, { recursive: true });
+
+    // Generate plugin.json
+    const pluginJson = {
+      name: pluginId,
+      description,
+      version: "1.0.0",
+      author: { name: "Awesome Copilot Community" },
+      repository: "https://github.com/github/awesome-copilot",
+      license: "MIT",
+      tags,
+      items: [],
+    };
+
+    fs.writeFileSync(
+      path.join(githubPluginDir, "plugin.json"),
+      JSON.stringify(pluginJson, null, 2) + "\n"
+    );
+
+    // Generate README.md
+    const readmeContent = `# ${displayName} Plugin
+
+${description}
+
+## Installation
+
+\`\`\`bash
+copilot plugin install ${pluginId}@awesome-copilot
+\`\`\`
+
+## What's Included
+
+_Add your plugin contents here._
+
+## Source
+
+This plugin is part of [Awesome Copilot](https://github.com/github/awesome-copilot).
+
+## License
+
+MIT
+`;
+
+    fs.writeFileSync(path.join(pluginDir, "README.md"), readmeContent);
+
+    console.log(`\n✅ Created plugin: ${pluginDir}`);
+    console.log("\n📝 Next steps:");
+    console.log(`1. Add agents, prompts, or instructions to plugins/${pluginId}/`);
+    console.log(`2. Update plugins/${pluginId}/.github/plugin/plugin.json to list your items`);
+    console.log(`3. Edit plugins/${pluginId}/README.md to describe your plugin`);
+    console.log("4. Run 'npm run build' to regenerate documentation");
+  } catch (error) {
+    console.error(`❌ Error creating plugin: ${error.message}`);
+    process.exit(1);
+  } finally {
+    rl.close();
+  }
+}
+
+createPlugin();