feat: add /scan-history skill to extract learnings from conversation history

[adrien-hearthands]

Summary

• Adds /scan-history skill that retrospectively analyzes Claude Code conversation history
• Identifies learnings that weren't captured during original sessions
• Presents candidates with confidence scores for user confirmation before creating skills
• Supports --user flag to save skills at user-level instead of project-level

Closes #15

How it works

1. Locates conversation history files (~/.claude/projects/<project>/*.jsonl)
2. Parses JSONL and extracts user/assistant exchanges
3. Identifies patterns indicating extractable knowledge (debugging sessions, error resolutions, workarounds)
4. Scores and ranks candidates by reusability, non-triviality, specificity
5. Presents summary table with candidates for user selection
6. Creates skill files for confirmed selections

Test plan

• Run /scan-history in a project with conversation history
• Verify it finds and lists conversation files
• Verify it presents candidate learnings with rationale
• Confirm a candidate and verify skill file is created correctly
• Test --user flag saves to ~/.claude/skills/

🤖 Generated with Claude Code

[adrien-hearthands]

'Needs refinement as tests shows the feature extraction is not great but it's a good base