feat: improve Mimir UX and documentation

[jb-thery]

Summary

• add kb doctor for setup/readiness/security next-step guidance
• make audio safer by default: Transformers WAV is default, Edge MP3 requires explicit --engine edge
• stop indexing the generated private/README.md helper file
• improve kb init and kb install-skill onboarding output
• add Getting Started, CLI Reference, Troubleshooting, and UX/DX Audit docs
• bump workspace packages and CLI to 0.4.2

Validation

• pnpm validate
• manual temp-repo UX flow: init -> doctor -> ingest -> doctor -> search
• manual audio guard check: MP3 output without --engine edge exits cleanly with a concise error
• release artifacts generated for 0.4.2

Notes

The CLI remains retrieval-only. local-hash is still documented as lexical/hash retrieval, not semantic search.