[Handoff] 25. Finish the public project guide

[code259]

Goal

Make the repository understandable and runnable without private context.

Why this matters

The README is the main entry point for people who will not inspect every file.

Work

• Lead with the problem, result, and limitations in plain language.
• Add a compact architecture diagram.
• Provide tested setup and run commands.
• Explain privacy, recovery, evaluation, and scaling choices.
• Link detailed technical and operational documents.

Done when

A reader can understand the project quickly, and a fresh checkout can follow every setup command.

Priority: Core — reserve time for this.