build: remove Nix flakes — Guix-only build policy

Owner ruling 2026-06-05: the Nix fallback is retired estate-wide. Removes
the root flake plus the echidnabot and sustainabot flakes (the latter in
both locations; sustainabot/nix/ held only flakes and is now gone). A Guix
manifest (guix.scm / guix channels) exists at every removed location.

https://claude.ai/code/session_016R5WsMHtpnD8bmrkjFNFmg

Author: claude