Support external plugins in marketplace.json generation (#876)

The marketplace currently only includes plugins that live as local
directories in plugins/. This makes it impossible to list plugins
hosted in external GitHub repos, npm packages, or other git URLs.

Add plugins/external.json as a hand-curated list of external plugin
entries following the Claude Code plugin marketplace spec. The
generate-marketplace script now reads this file and merges external
entries as-is into the generated marketplace.json, sorted by name.

Changes:
- Add plugins/external.json (empty array, ready for entries)
- Update eng/generate-marketplace.mjs to load, merge, and sort
  external plugins; warn on duplicate names; log counts
- Document the external plugin workflow in CONTRIBUTING.md and
  AGENTS.md

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

eng/generate-marketplace.mjs

@@ -5,8 +5,82 @@ import path from "path";
 import { ROOT_FOLDER } from "./constants.mjs";
 
 const PLUGINS_DIR = path.join(ROOT_FOLDER, "plugins");
+const EXTERNAL_PLUGINS_FILE = path.join(ROOT_FOLDER, "plugins", "external.json");
 const MARKETPLACE_FILE = path.join(ROOT_FOLDER, ".github/plugin", "marketplace.json");
 
+/**
+ * Validate an external plugin entry has required fields and a non-local source
+ * @param {object} plugin - External plugin entry
+ * @param {number} index - Index in the array (for error messages)
+ * @returns {string[]} - Array of validation error messages
+ */
+function validateExternalPlugin(plugin, index) {
+  const errors = [];
+  const prefix = `external.json[${index}]`;
+
+  if (!plugin.name || typeof plugin.name !== "string") {
+    errors.push(`${prefix}: "name" is required and must be a string`);
+  }
+  if (!plugin.description || typeof plugin.description !== "string") {
+    errors.push(`${prefix}: "description" is required and must be a string`);
+  }
+  if (!plugin.version || typeof plugin.version !== "string") {
+    errors.push(`${prefix}: "version" is required and must be a string`);
+  }
+
+  if (!plugin.source) {
+    errors.push(`${prefix}: "source" is required`);
+  } else if (typeof plugin.source === "string") {
+    errors.push(`${prefix}: "source" must be an object (local file paths are not allowed for external plugins)`);
+  } else if (typeof plugin.source === "object") {
+    if (!plugin.source.source) {
+      errors.push(`${prefix}: "source.source" is required (e.g. "github", "url", "npm", "pip")`);
+    }
+  } else {
+    errors.push(`${prefix}: "source" must be an object`);
+  }
+
+  return errors;
+}
+
+/**
+ * Read external plugin entries from external.json
+ * @returns {Array} - Array of external plugin entries (merged as-is)
+ */
+function readExternalPlugins() {
+  if (!fs.existsSync(EXTERNAL_PLUGINS_FILE)) {
+    return [];
+  }
+
+  try {
+    const content = fs.readFileSync(EXTERNAL_PLUGINS_FILE, "utf8");
+    const plugins = JSON.parse(content);
+    if (!Array.isArray(plugins)) {
+      console.warn("Warning: external.json must contain an array");
+      return [];
+    }
+
+    // Validate each entry
+    let hasErrors = false;
+    for (let i = 0; i < plugins.length; i++) {
+      const errors = validateExternalPlugin(plugins[i], i);
+      if (errors.length > 0) {
+        errors.forEach(e => console.error(`Error: ${e}`));
+        hasErrors = true;
+      }
+    }
+    if (hasErrors) {
+      console.error("Error: external.json contains invalid entries");
+      process.exit(1);
+    }
+
+    return plugins;
+  } catch (error) {
+    console.error(`Error reading external.json: ${error.message}`);
+    return [];
+  }
+}
+
 /**
  * Read plugin metadata from plugin.json file
  * @param {string} pluginDir - Path to plugin directory
@@ -67,6 +141,25 @@ function generateMarketplace() {
     }
   }
 
+  // Read external plugins and merge as-is
+  const externalPlugins = readExternalPlugins();
+  if (externalPlugins.length > 0) {
+    console.log(`\nFound ${externalPlugins.length} external plugins`);
+
+    // Warn on duplicate names
+    const localNames = new Set(plugins.map(p => p.name));
+    for (const ext of externalPlugins) {
+      if (localNames.has(ext.name)) {
+        console.warn(`Warning: external plugin "${ext.name}" has the same name as a local plugin`);
+      }
+      plugins.push(ext);
+      console.log(`✓ Added external plugin: ${ext.name}`);
+    }
+  }
+
+  // Sort all plugins by name (case-insensitive)
+  plugins.sort((a, b) => a.name.localeCompare(b.name, undefined, { sensitivity: "base" }));
+
   // Create marketplace.json structure
   const marketplace = {
     name: "awesome-copilot",
@@ -91,7 +184,7 @@ function generateMarketplace() {
   // Write marketplace.json
   fs.writeFileSync(MARKETPLACE_FILE, JSON.stringify(marketplace, null, 2) + "\n");
 
-  console.log(`\n✓ Successfully generated marketplace.json with ${plugins.length} plugins`);
+  console.log(`\n✓ Successfully generated marketplace.json with ${plugins.length} plugins (${plugins.length - externalPlugins.length} local, ${externalPlugins.length} external)`);
   console.log(`  Location: ${MARKETPLACE_FILE}`);
 }