Document BLU planning repository

[mattleech]

What changed

• explains that this is a planning-only repository for possible Blutopia automation
• summarizes the proposed rTorrent, ZNC/IRC, and storage design
• documents the current status, planned environment, and related repositories
• adds security and compliance guidance for this public repository

Why

The repository contained only a detailed plan, with no top-level explanation that it is not yet a runnable application.

Validation

• verified all local Markdown links target existing documents or repositories
• git diff --check