Migrate extension metadata to plugin.json and enforce conventions (#2177)

* Remove pluginRoots property from marketplace.json

The pluginRoots property is not used by install tooling and was only
informational about the extension/plugin source directories. Removing it
simplifies the marketplace.json structure while maintaining all functionality.

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

* Migrate java-modernization-studio to plugin.json and update validation workflow

- Create .github/plugin/plugin.json for java-modernization-studio extension
- Remove legacy canvas.json from java-modernization-studio
- Update validate-canvas-extensions.yml workflow to check for plugin.json instead of canvas.json
- Update workflow to trigger on .schemas/plugin.schema.json changes (instead of canvas.schema.json)
- Remove schema validation logic that relied on canvas.schema.json
- All 12 extensions now use plugin.json for metadata

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

* Add extensions field to all extension plugin.json files

Per https://github.com/github/copilot-agent-runtime/pull/9929, plugins that ship
extensions need to include an extensions field specifying where the extension code
is located. All 12 extensions now have extensions set to '.' to reference the
current directory where extension.mjs is located.

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

* Enforce convention-based extension metadata and remove x-awesome-copilot

- Remove x-awesome-copilot.screenshots from all extension plugin.json files
- Enforce logo=assets/preview.png convention for all extensions
- Enforce extensions=. per copilot-agent-runtime#9929
- Update validate-plugins.mjs to enforce conventions
- Update validate-canvas-extensions.yml workflow with convention checks
- Update AGENTS.md and CONTRIBUTING.md documentation

All 12 extensions validated successfully.

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

* Potential fix for pull request finding

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

* Use standard plugin validation for extensions

Remove the custom extension schema and schema validation helper, and
validate extension plugin.json files through the existing plugin validator
instead. Update workflows to stop depending on the removed schema.

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

---------

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

eng/generate-marketplace.mjs

@@ -6,6 +6,7 @@ import { ROOT_FOLDER } from "./constants.mjs";
 import { readExternalPlugins } from "./external-plugin-validation.mjs";
 
 const PLUGINS_DIR = path.join(ROOT_FOLDER, "plugins");
+const EXTENSIONS_DIR = path.join(ROOT_FOLDER, "extensions");
 const MARKETPLACE_FILE = path.join(ROOT_FOLDER, ".github/plugin", "marketplace.json");
 
 /**
@@ -30,43 +31,58 @@ function readPluginMetadata(pluginDir) {
   }
 }
 
+function collectLocalPluginsFromRoot(rootDir, sourcePrefix, includeEntry = () => true) {
+  if (!fs.existsSync(rootDir)) {
+    return [];
+  }
+
+  const entries = fs.readdirSync(rootDir, { withFileTypes: true })
+    .filter(entry => entry.isDirectory())
+    .filter(entry => includeEntry(entry.name))
+    .map(entry => entry.name)
+    .sort();
+
+  const plugins = [];
+  for (const dirName of entries) {
+    const pluginPath = path.join(rootDir, dirName);
+    const metadata = readPluginMetadata(pluginPath);
+
+    if (!metadata) {
+      continue;
+    }
+
+    plugins.push({
+      name: metadata.name,
+      source: `${sourcePrefix}/${dirName}`,
+      description: metadata.description,
+      version: metadata.version || "1.0.0"
+    });
+  }
+
+  return plugins;
+}
+
 /**
  * Generate marketplace.json from plugin directories
  */
 function generateMarketplace() {
   console.log("Generating marketplace.json...");
 
-  if (!fs.existsSync(PLUGINS_DIR)) {
-    console.error(`Error: Plugins directory not found at ${PLUGINS_DIR}`);
+  if (!fs.existsSync(PLUGINS_DIR) && !fs.existsSync(EXTENSIONS_DIR)) {
+    console.error(`Error: Neither plugins directory (${PLUGINS_DIR}) nor extensions directory (${EXTENSIONS_DIR}) was found`);
     process.exit(1);
   }
 
-  // Read all plugin directories
-  const pluginDirs = fs.readdirSync(PLUGINS_DIR, { withFileTypes: true })
-    .filter(entry => entry.isDirectory())
-    .map(entry => entry.name)
-    .sort();
+  const plugins = [
+    ...collectLocalPluginsFromRoot(PLUGINS_DIR, "plugins"),
+    ...collectLocalPluginsFromRoot(
+      EXTENSIONS_DIR,
+      "extensions",
+      (entryName) => fs.existsSync(path.join(EXTENSIONS_DIR, entryName, "extension.mjs"))
+    )
+  ];
 
-  console.log(`Found ${pluginDirs.length} plugin directories`);
-
-  // Read metadata for each plugin
-  const plugins = [];
-  for (const dirName of pluginDirs) {
-    const pluginPath = path.join(PLUGINS_DIR, dirName);
-    const metadata = readPluginMetadata(pluginPath);
-
-    if (metadata) {
-      plugins.push({
-        name: metadata.name,
-        source: dirName,
-        description: metadata.description,
-        version: metadata.version || "1.0.0"
-      });
-      console.log(`✓ Added plugin: ${metadata.name}`);
-    } else {
-      console.log(`✗ Skipped: ${dirName} (no valid plugin.json)`);
-    }
-  }
+  console.log(`Found ${plugins.length} local plugin manifests`);
 
   // Read external plugins and merge as-is
   const { plugins: externalPlugins, errors: externalErrors, warnings: externalWarnings } = readExternalPlugins({
@@ -96,8 +112,7 @@ function generateMarketplace() {
     name: "awesome-copilot",
     metadata: {
       description: "Community-driven collection of GitHub Copilot plugins, agents, prompts, and skills",
-      version: "1.0.0",
-      pluginRoot: "./plugins"
+      version: "1.0.0"
     },
     owner: {
       name: "GitHub",