Migrate plugins and canvas extensions to Agent Plugins spec (#2546)

* feat: migrate plugins and extensions to Agent Plugins v1.0.0 spec

- Add \ to all 69 curated plugin manifests
- Migrate all 18 extension manifests: add \, move logo into
  xtensions.com.github.copilot.logo namespace, remove top-level
  logo and string xtensions: '.'
- Update eng/validate-plugins.mjs: require \, validate
  namespace-keyed extensions object for canvas extensions, widen
  name pattern to allow dots (spec §5.5, max 64 chars)
- Update eng/materialize-plugins.mjs: emit spec-clean served manifests
  (only spec fields: \, name, version, description, author,
  homepage, repository, license, keywords, extensions)
- Update eng/generate-website-data.mjs: read logo from namespace
  with fallback to top-level logo for compatibility
- Update eng/create-plugin.mjs: scaffold emits \
- Add .github/workflows/validate-plugins.yml: blocking CI for PRs
  touching plugins/** or extensions/**
- Add spec compliance check to external plugin quality gates:
  non-blocking warnings with ✅/⚠️/🛑 emoji legend
- Update AGENTS.md: document new extension manifest shape,
  add \ to plugin checklist

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 8f3a88cb-e01e-4760-8125-460490dc1a76

* refactor: consolidate canvas extension plugins

- Move all extension plugin manifests from extensions/<name> to plugins/<name>
- Keep extensions/<name> as reusable source only
- Remove standalone extension discovery from marketplace and website plugin catalogs
- Auto-bundle same-name extension sources during materialization
- Add build-only extensions.json references for sharing extensions across plugins
- Remove x-awesome-copilot extension metadata support
- Update validation and contributor documentation

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 8f3a88cb-e01e-4760-8125-460490dc1a76

* feat: add canvas extension scaffolding skill

- Add repo-local skill for creating canvas extension sources
- Generate spec-compliant plugin manifests under plugins/
- Support registering reusable extensions with multiple plugins
- Remove guidance for extension-local plugin manifests and custom fields

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 8f3a88cb-e01e-4760-8125-460490dc1a76

* fix: align extension namespaces with current guidance

- Use each extension ID as its manifest namespace key
- Update validation and website generation to resolve extension-specific namespaces
- Upsert plugin validation PR comments using the existing repository pattern

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 8f3a88cb-e01e-4760-8125-460490dc1a76

* fix: use Copilot extension namespace

- Adopt com.github.copilot for all canvas extension manifests
- Require the namespace during validation and website generation
- Update extension scaffolding guidance

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 8f3a88cb-e01e-4760-8125-460490dc1a76

* docs: regenerate plugin catalog after merge

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 8f3a88cb-e01e-4760-8125-460490dc1a76

* refactor(plugins): move manifests to plugin roots

Use root plugin.json manifests and namespaced extension directories throughout local tooling, validation, generation, and contributor documentation. Restore materialize-plugins.mjs line breaks so the source remains readable in GitHub.

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

Copilot-Session: 8f3a88cb-e01e-4760-8125-460490dc1a76

* feat(plugins): migrate manifests to namespaced composition

Move repository composition metadata under com.github.awesome-copilot, materialize reusable extensions into the plugin extensions directory, and improve contributor and PR validation guidance.

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

Copilot-Session: 8f3a88cb-e01e-4760-8125-460490dc1a76

* fix(validation): address plugin review findings

Restore executable build scripts, validate namespaced manifests and hook directories, improve README item counts, and manage validation comments across reruns.

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

Copilot-Session: 8f3a88cb-e01e-4760-8125-460490dc1a76

* Potential fix for pull request finding

Co-authored-by: Copilot Autofix powered by AI <175728472+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>
Copilot-Session: 8f3a88cb-e01e-4760-8125-460490dc1a76

eng/update-readme.mjs

@@ -29,6 +29,7 @@ import {
 
 const __filename = fileURLToPath(import.meta.url);
 const __dirname = dirname(__filename);
+const EXTENSIONS_DIR = path.join(ROOT_FOLDER, "extensions");
 
 // Cache of MCP registry server names (lower-cased) fetched from the API
 let MCP_REGISTRY_SET = null;
@@ -715,7 +716,7 @@ function generateUnifiedModeSection(cfg) {
  * Read and parse a plugin.json file from a plugin directory.
  */
 function readPluginJson(pluginDir) {
-  const jsonPath = path.join(pluginDir, ".github/plugin", "plugin.json");
+  const jsonPath = path.join(pluginDir, "plugin.json");
   if (!fs.existsSync(jsonPath)) return null;
   try {
     return JSON.parse(fs.readFileSync(jsonPath, "utf-8"));
@@ -786,10 +787,23 @@ function generatePluginsSection(pluginsDir) {
   for (const entry of sortedEntries) {
     const { plugin, dir, name, isFeatured } = entry;
     const description = formatTableCell(plugin.description || "No description");
+    const composition = plugin.extensions?.["com.github.awesome-copilot"] || {};
+    const extensionReferences = Array.isArray(composition.extensions)
+      ? composition.extensions.length
+      : 0;
+    const implicitExtension =
+      fs.existsSync(path.join(EXTENSIONS_DIR, entry.pluginId, "extension.mjs")) &&
+      !(Array.isArray(composition.extensions) && composition.extensions.some(
+        (reference) => reference === `./extensions/${entry.pluginId}`
+      ))
+      ? 1
+      : 0;
     const itemCount =
-      (plugin.agents || []).length +
-      (plugin.commands || []).length +
-      (plugin.skills || []).length;
+      (composition.agents || []).length +
+      (composition.commands || []).length +
+      (composition.skills || []).length +
+      extensionReferences +
+      implicitExtension;
     const keywords = plugin.keywords ? plugin.keywords.join(", ") : "";
 
     const link = `../plugins/${dir}/README.md`;
@@ -842,10 +856,23 @@ function generateFeaturedPluginsSection(pluginsDir) {
             plugin.description || "No description"
           );
           const keywords = plugin.keywords ? plugin.keywords.join(", ") : "";
+          const composition = plugin.extensions?.["com.github.awesome-copilot"] || {};
+          const extensionReferences = Array.isArray(composition.extensions)
+            ? composition.extensions.length
+            : 0;
+          const implicitExtension =
+            fs.existsSync(path.join(EXTENSIONS_DIR, name, "extension.mjs")) &&
+            !(Array.isArray(composition.extensions) && composition.extensions.some(
+              (reference) => reference === `./extensions/${name}`
+            ))
+            ? 1
+            : 0;
           const itemCount =
-            (plugin.agents || []).length +
-            (plugin.commands || []).length +
-            (plugin.skills || []).length;
+            (composition.agents || []).length +
+            (composition.commands || []).length +
+            (composition.skills || []).length +
+            extensionReferences +
+            implicitExtension;
 
           return {
             dir,