[codex] Migrate Cachix usage to Attic

[zah]

Summary

• replace Cachix cache/deploy usage with Attic cache configuration
• route Nix setup through the shared Attic-aware setup action or repo Attic variables
• remove legacy Cachix push/setup paths where this repo had them

Validation

• parsed changed GitHub workflow YAML with yq
• parsed changed Nix files with nix-instantiate --parse
• ran git diff --check