docs: clarify package readmes

[jb-thery]

Summary

• make each package README sufficiently descriptive for npm while keeping the root README canonical
• explain what @jcode.labs/mimir does, what it is for, and how to start quickly
• explain what @jcode.labs/mimir-tts does, its offline and Edge TTS paths, and how to start quickly
• fix the synthetic demo README path from the repository root
• prepare version 0.4.4 so npm can display the updated package READMEs after publish

Validation

• pnpm validate
• git diff --check
• inspected generated 0.4.4 tarballs for @jcode.labs/mimir and @jcode.labs/mimir-tts README content
• inspected generated @jcode.labs/mimir tarball for the corrected sovereign-rag-demo README path