ci(workflows): add skill sync and validation workflows

- Add sync-claude-plugin workflow to auto-update marketplace.json when skills change
- Add validate-skill workflow to validate SKILL.md files on push/PR
- Add sync-marketplace.js script for skill discovery and count updates

.github/scripts/sync-marketplace.js

@@ -0,0 +1,67 @@
+#!/usr/bin/env node
+/**
+ * Sync marketplace.json with skills directory.
+ *
+ * Scans the skills/ directory for valid skills (directories containing SKILL.md)
+ * and updates marketplace.json to match.
+ */
+
+const fs = require("fs");
+const path = require("path");
+
+const SKILLS_DIR = "skills";
+const MARKETPLACE_FILE = ".claude-plugin/marketplace.json";
+
+function getSkillsFromDirectory() {
+  if (!fs.existsSync(SKILLS_DIR)) {
+    return [];
+  }
+
+  return fs
+    .readdirSync(SKILLS_DIR, { withFileTypes: true })
+    .filter((entry) => {
+      if (!entry.isDirectory()) return false;
+      const skillFile = path.join(SKILLS_DIR, entry.name, "SKILL.md");
+      return fs.existsSync(skillFile);
+    })
+    .map((entry) => `./${SKILLS_DIR}/${entry.name}`)
+    .sort();
+}
+
+function updateSkillCount(description, count) {
+  return description.replace(/\d+ marketing skills/, `${count} marketing skills`);
+}
+
+function main() {
+  const currentSkills = getSkillsFromDirectory();
+
+  const marketplace = JSON.parse(fs.readFileSync(MARKETPLACE_FILE, "utf8"));
+  const plugin = marketplace.plugins[0];
+  const existingSkills = plugin.skills || [];
+
+  // Check if update needed
+  if (JSON.stringify(currentSkills) === JSON.stringify(existingSkills)) {
+    console.log("marketplace.json is already in sync");
+    return;
+  }
+
+  // Update skills list
+  plugin.skills = currentSkills;
+
+  // Update description with new count
+  plugin.description = updateSkillCount(plugin.description, currentSkills.length);
+
+  // Write updated marketplace.json
+  fs.writeFileSync(MARKETPLACE_FILE, JSON.stringify(marketplace, null, 2) + "\n");
+
+  // Report changes
+  const added = currentSkills.filter((s) => !existingSkills.includes(s));
+  const removed = existingSkills.filter((s) => !currentSkills.includes(s));
+
+  if (added.length) console.log(`Added: ${added.join(", ")}`);
+  if (removed.length) console.log(`Removed: ${removed.join(", ")}`);
+
+  console.log(`Updated marketplace.json (${currentSkills.length} skills)`);
+}
+
+main();