ci(nix): automate weekly flake.lock updates on linux/rust

[YuriNek0]

Linked Issues

Closes #550

What Changed

• Added workflow: .github/workflows/ci-nix-flake-update.yml
• Runs on weekly schedule + manual dispatch
• Runs:
  • nix flake update
  • detects whether flake.lock changed
  • nix flake check --no-build only if changed
  • nix build .#default only if changed
• Commits and pushes flake.lock back to linux/rust only if changed.
• Keeps PR automation block commented out (can switch between direct-commit and PR mode)

Notes

• No-op runs do not commit anything.
• Commit happens only after update + check + build steps pass.