feat: extract skills clusters in mem history

[IRONICBo]

Background

Add AI‑driven feature clustering and skill summaries to MemoV history, cache them in SQLite, and surface them in the UI.

Changes

• Add SkillsDB (SQLite) to store features, commit mappings, and skill docs.
• Add /api/skills (read cached skills) and /api/skills/refresh (generate clusters + skill summaries via OpenAI with validation/retries).
• Add Skills view UI (cards, filters, pagination, detail modal, coachmarks) and new memov-icon.svg.
• Ignore .memignore and make git subprocess output decoding resilient across platforms.

[IRONICBo]

New skills view.