feat: add lesson-learned skill

[mattwhite8]

Summary

• Adds a new lesson-learned skill that analyzes recent git history and extracts software engineering takeaways grounded in named principles (SOLID, DRY, KISS, YAGNI, etc.)
• Includes curated reference files for SE principles and anti-patterns to ensure specific, code-grounded analysis
• Registers the skill in marketplace.json and the root README

Test plan

• Install skill via cp -r skills/lesson-learned ~/.claude/skills/
• Navigate to a git repo with recent commits
• Trigger with "what is the lesson here?" and verify it produces a structured, code-specific takeaway
• Verify marketplace.json is valid JSON
• Verify README table renders correctly

🤖 Generated with Claude Code

[leonardocouy]

Hey @mattwhite8, thanks for the contribution! Really cool skill — I love the idea of reflecting on code changes and extracting SE lessons from actual diffs. The reference files for principles and anti-patterns are well curated too.

merged!