feat(skills): track last used timestamp

[steve-wecompare]

Summary

• Add nullable skills.last_used_at column with Drizzle migration/snapshot
• Include last_used_at in skill list/discovery output
• Update successful skill reads through MCP/API to refresh last_used_at
• Add integration assertions for list/read timestamp behavior

Verification

• npm run check passes with existing Svelte warnings
• npm run build passes with existing Svelte warnings
• npm test -- --run tests/integration/skills.test.ts tests/integration/mcp.test.ts could not run here because Testcontainers cannot find a working container runtime