Add first-class Grok Build harness support

Emit .grok skills, agents, and PostToolUse/Stop hooks; wire the CLI
installer and downloads; fix the plugin install path to #plugin; and
document Grok in HARNESSES.md and README.

AI assistance: written with Grok Build.

scripts/build.js

@@ -462,11 +462,15 @@ This folder contains skills for all supported tools:
   .codex/     -> Codex custom agents (Codex skills use .agents/)
   .agents/    -> Codex CLI
   .github/    -> GitHub Copilot
+  .grok/      -> Grok Build
   .kiro/      -> Kiro
   .opencode/  -> OpenCode
   .pi/        -> Pi
   .trae-cn/   -> Trae China
   .trae/      -> Trae International
+  .rovodev/   -> Rovo Dev
+  .vibe/      -> Mistral Vibe
+  .qoder/     -> Qoder
 
 To install, copy the relevant folder(s) into your project root.
 For Codex, repo and user skill installs come from .agents/skills.
@@ -590,18 +594,21 @@ async function build() {
 
     console.log(`📋 Synced skills to: ${syncConfigs.map(p => p.configDir).join(', ')}`);
 
-    // Build the Claude Code plugin subtree at ./plugin/.
-    // The Claude Code marketplace is configured with `source: "./plugin"`, so
-    // the plugin cache only copies this slim directory (~0.3 MB) instead of
-    // the entire monorepo (~291 MB on the previous "./" source). The harness
-    // dirs above stay where they are because `npx skills add pbakaus/impeccable`
-    // reads them directly from the GitHub repo at install time.
+    // Build the shared plugin subtree at ./plugin/.
+    // Claude Code marketplace is configured with `source: "./plugin"`, so the
+    // plugin cache only copies this slim directory (~0.3 MB) instead of the
+    // entire monorepo. Grok Build installs the same subtree via
+    // `grok plugin install pbakaus/impeccable#plugin --trust` (or the
+    // marketplace source). The harness dirs above stay where they are because
+    // `npx skills add pbakaus/impeccable` reads them from the GitHub repo.
     const pluginRoot = path.join(ROOT_DIR, 'plugin');
     const pluginManifestDir = path.join(pluginRoot, '.claude-plugin');
+    const grokPluginManifestDir = path.join(pluginRoot, '.grok-plugin');
     const pluginSkillsDir = path.join(pluginRoot, 'skills');
     const pluginAgentsDir = path.join(pluginRoot, 'agents');
     const pluginHooksDir = path.join(pluginRoot, 'hooks');
     if (fs.existsSync(pluginManifestDir)) fs.rmSync(pluginManifestDir, { recursive: true });
+    if (fs.existsSync(grokPluginManifestDir)) fs.rmSync(grokPluginManifestDir, { recursive: true });
     if (fs.existsSync(pluginSkillsDir)) fs.rmSync(pluginSkillsDir, { recursive: true });
     if (fs.existsSync(pluginAgentsDir)) fs.rmSync(pluginAgentsDir, { recursive: true });
     if (fs.existsSync(pluginHooksDir)) fs.rmSync(pluginHooksDir, { recursive: true });
@@ -636,6 +643,26 @@ async function build() {
       JSON.stringify(pluginManifest, null, 2) + '\n',
     );
 
+    // Native Grok plugin manifest. Grok also reads `.claude-plugin/`; dual
+    // manifests keep both marketplaces and `grok plugin validate` happy when
+    // Claude compat is disabled.
+    // https://docs.x.ai/build/features/skills-plugins-marketplaces
+    const grokPluginManifest = {
+      name: pluginManifest.name,
+      version: pluginManifest.version,
+      description: pluginManifest.description,
+      author: pluginManifest.author,
+      homepage: pluginManifest.homepage,
+      repository: pluginManifest.repository,
+      license: pluginManifest.license || 'MIT',
+      keywords: ['design', 'frontend', 'ui', 'ux', 'skills', 'hooks'],
+    };
+    fs.mkdirSync(grokPluginManifestDir, { recursive: true });
+    fs.writeFileSync(
+      path.join(grokPluginManifestDir, 'plugin.json'),
+      JSON.stringify(grokPluginManifest, null, 2) + '\n',
+    );
+
     const claudeSkillsSrc = path.join(DIST_DIR, 'claude-code', '.claude', 'skills', 'impeccable');
     if (fs.existsSync(claudeSkillsSrc)) {
       fs.mkdirSync(pluginSkillsDir, { recursive: true });
@@ -646,17 +673,18 @@ async function build() {
       copyDirSync(claudeAgentsSrc, pluginAgentsDir);
     }
 
-    // Ship the design detector as a plugin-packaged hook. Claude Code
-    // auto-discovers `hooks/hooks.json` at the plugin root, so marketplace /
-    // `/plugin install` users get the PostToolUse hook without it being merged
-    // into their project `.claude/settings.json` (that path is the CLI's job).
+    // Ship the design detector as a plugin-packaged hook. Claude Code and
+    // Grok Build both auto-discover `hooks/hooks.json` at the plugin root
+    // (Grok aliases CLAUDE_PLUGIN_ROOT → GROK_PLUGIN_ROOT), so marketplace /
+    // plugin-install users get PostToolUse + Stop without merging into project
+    // settings (that path remains the CLI's job for project-scoped installs).
     fs.mkdirSync(pluginHooksDir, { recursive: true });
     fs.writeFileSync(
       path.join(pluginHooksDir, 'hooks.json'),
       JSON.stringify(buildClaudePluginHooksManifest(), null, 2) + '\n',
     );
 
-    console.log('📦 Built Claude Code plugin subtree at ./plugin/');
+    console.log('📦 Built Claude Code / Grok Build plugin subtree at ./plugin/');
   } else {
     console.log('📋 Skipped root harness and plugin sync (--skip-root-sync)');
   }