Documentation: Finalize installation, permissions, and limitations

[Millbot-AI]

Goal

Prepare concise release documentation for the smallest verified Backtrack MVP.

Acceptance criteria

• Explain unpacked installation in Brave and Chrome.
• Describe the architecture and current development status.
• Justify every requested permission and the data it could expose.
• Document restricted pages and known limitations.
• Explain local debugging without telemetry.
• State the verified browser and macOS versions.
• Keep unsupported behavior and the Phase 1 evidence visible.