feat: add dynamic sitemap for Google Search Console indexing

- Generate sitemap.xml with all skill URLs for search engine discovery
- Return empty sitemap on mirror servers to prevent duplicate indexing

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/db/src/queries.ts

@@ -399,6 +399,20 @@ export const skillQueries = {
     return result[0]?.count ?? 0;
   },
 
+  /**
+   * Get all skills for sitemap generation (lightweight: id, updatedAt, githubOwner only)
+   */
+  getAllForSitemap: async (db: DB) => {
+    return db
+      .select({
+        id: skills.id,
+        updatedAt: skills.updatedAt,
+        githubOwner: skills.githubOwner,
+      })
+      .from(skills)
+      .where(and(eq(skills.isBlocked, false), eq(skills.sourceFormat, 'skill.md')));
+  },
+
   /**
    * Get site engagement level for adaptive algorithm
    * Returns: 'cold_start' | 'growth' | 'mature'