sweep: delete Nix estate-wide (63 repos + 53 .envrc blocks + 13 CI files)

[hyperpolymath]

Goal

Remove Nix per the Guix-only policy (standards#405). Run via the Elixir megasweep, in parallel.

Scope (scan 2026-06-22)

• 63 repos = 'BOTH' (Nix + Guix present) → safe to delete Nix: remove flake.nix/flake.lock/*.nix (~181 files total across 66 repos).
• 53 repos with a use flake block in .envrc → remove the Nix block.
• 13 CI workflow files with real Nix usage (nix-installer/determinate/nix build) → strip.

Constraints

• Do NOT touch the 3 'NIX_NO_GUIX' repos here (separate issue — Guix must be built first).
• Staged, signed, owner-approves push. Cut branches from fresh origin/main. No licence edits.
• Lists in dev-notes/ nix scan (nix-files.txt, envrc-nix.txt, ci-nix.txt).