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>

skills/email-sequence/references/copy-guidelines.md

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+# Email Copy Guidelines
+
+## Structure
+
+1. **Hook**: First line grabs attention
+2. **Context**: Why this matters to them
+3. **Value**: The useful content
+4. **CTA**: What to do next
+5. **Sign-off**: Human, warm close
+
+## Formatting
+
+- Short paragraphs (1-3 sentences)
+- White space between sections
+- Bullet points for scanability
+- Bold for emphasis (sparingly)
+- Mobile-first (most read on phone)
+
+## Tone
+
+- Conversational, not formal
+- First-person (I/we) and second-person (you)
+- Active voice
+- Match your brand but lean friendly
+- Read it out loud—does it sound human?
+
+## Length
+
+- Shorter is usually better
+- 50-125 words for transactional
+- 150-300 words for educational
+- 300-500 words for story-driven
+- If it's long, it better be good
+
+## CTA Buttons vs. Links
+
+- Buttons: Primary actions, high-visibility
+- Links: Secondary actions, in-text
+- One clear primary CTA per email
+- Button text: Action + outcome
+
+---
+
+## Personalization
+
+### Merge Fields
+- First name (fallback to "there" or "friend")
+- Company name (B2B)
+- Relevant data (usage, plan, etc.)
+
+### Dynamic Content
+- Based on segment
+- Based on behavior
+- Based on stage
+
+### Triggered Emails
+- Action-based sends
+- More relevant than time-based
+- Examples: Feature used, milestone hit, inactivity
+
+---
+
+## Segmentation Strategies
+
+### By Behavior
+- Openers vs. non-openers
+- Clickers vs. non-clickers
+- Active vs. inactive
+
+### By Stage
+- Trial vs. paid
+- New vs. long-term
+- Engaged vs. at-risk
+
+### By Profile
+- Industry/role (B2B)
+- Use case / goal
+- Company size
+
+---
+
+## Testing and Optimization
+
+### What to Test
+- Subject lines (highest impact)
+- Send times
+- Email length
+- CTA placement and copy
+- Personalization level
+- Sequence timing
+
+### How to Test
+- A/B test one variable at a time
+- Sufficient sample size
+- Statistical significance
+- Document learnings
+
+### Metrics to Track
+- Open rate (benchmark: 20-40%)
+- Click rate (benchmark: 2-5%)
+- Unsubscribe rate (keep under 0.5%)
+- Conversion rate (specific to sequence goal)
+- Revenue per email (if applicable)