feat: introduce dashboard skills - #785

Summary

• Add read-only bundled skill storage for package/app-provided skills.
• Add package-owned Mosaic dashboard skills plus deterministic selection and prompt rendering.
• Add SQLRooms CLI dashboard/worksheet skill roots, trace fixtures, and agent wiring that records applied {id, rootId} metadata.
• Document the skills pattern in package READMEs and contributing docs.

Validation

• pnpm install
• pnpm build
• pnpm --filter @sqlrooms/ai test --runInBand -- bundledSkillStorage.test.ts
• pnpm --filter @sqlrooms/mosaic test --runInBand -- dashboardSkills.test.ts
• pnpm --filter sqlrooms-cli-app test --runInBand -- dashboardSkills.test.ts
• pnpm --filter sqlrooms-cli-app build
• Push hook: knip, turbo typecheck, madge --circular packages/*/src/index.ts, turbo test

Notes

• Obsidian plan stages were updated to implemented.
• Related Obsidian assessment notes were linked back to the dashboard skills introduction plan.