Optimize skill files for AI agent use with progressive disclosure

- Fix marketplace.json: add 2 missing skills (content-strategy, product-marketing-context)
- Refactor 10 skills over 500 lines to use references/ folders:
  - email-sequence: 926 → 291 lines
  - social-content: 809 → 276 lines
  - competitor-alternatives: 750 → 253 lines
  - pricing-strategy: 712 → 226 lines
  - programmatic-seo: 628 → 235 lines
  - referral-program: 604 → 239 lines
  - schema-markup: 598 → 175 lines
  - free-tool-strategy: 576 → 176 lines
  - paywall-upgrade-cro: 572 → 224 lines
  - marketing-ideas: 566 → 165 lines

Each skill now has core workflow in SKILL.md (<500 lines) with detailed
content in references/ folder for progressive disclosure.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.claude-plugin/marketplace.json

@@ -12,13 +12,14 @@
   "plugins": [
     {
       "name": "marketing-skills",
-      "description": "23 marketing skills for technical marketers and founders: CRO, copywriting, SEO, paid ads, pricing strategy, referral programs, and more",
+      "description": "25 marketing skills for technical marketers and founders: CRO, copywriting, SEO, paid ads, pricing strategy, referral programs, and more",
       "source": "./",
       "strict": false,
       "skills": [
         "./skills/ab-test-setup",
         "./skills/analytics-tracking",
         "./skills/competitor-alternatives",
+        "./skills/content-strategy",
         "./skills/copy-editing",
         "./skills/copywriting",
         "./skills/email-sequence",
@@ -33,6 +34,7 @@
         "./skills/paywall-upgrade-cro",
         "./skills/popup-cro",
         "./skills/pricing-strategy",
+        "./skills/product-marketing-context",
         "./skills/programmatic-seo",
         "./skills/referral-program",
         "./skills/schema-markup",