feat: add lead-magnets skill (#177)

* feat: add lead-magnets skill

Covers strategy for creating, gating, promoting, and measuring
downloadable/content-based lead magnets (ebooks, checklists, cheat
sheets, templates, swipe files, mini-courses, quizzes, webinars).

Includes format-guide and benchmarks reference files. Adds cross-
references in free-tool-strategy, email-sequence, and popup-cro.

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

* fix: align H1 title with skill name

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

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Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

skills/popup-cro/SKILL.md

@@ -447,6 +447,7 @@ Ideas to A/B test with expected outcomes
 
 ## Related Skills
 
+- **lead-magnets**: For planning lead magnets to promote via popups
 - **form-cro**: For optimizing the form inside the popup
 - **page-cro**: For the page context around popups
 - **email-sequence**: For what happens after popup conversion