feat: mo update critical flake inputs auto pr

[samrose]

Summary

This PR adds automated monthly and bimonthly workflows to keep flake inputs up to date
Changes

Monthly workflow for critical inputs (update-flake-lock.yml)

• Runs on the 1st of every month (and via manual dispatch)
• Updates the four inputs that track fast-moving upstream targets:
  • nixpkgs (nixos-unstable channel)
  • rust-overlay (Rust toolchain releases)
  • multigres (upstream project, flake = false)
  • nix-eval-jobs (build tooling)
• Opens a PR against develop using peter-evans/create-pull-request
• No GitHub App required, uses the built-in GITHUB_TOKEN

Bimonthly workflow for non-critical inputs (update-flake-lock-non-critical.yml)

• Runs on the 1st of every other month (and via manual dispatch)

• Updates stable utility libraries that change infrequently:

  • devshell
  • flake-parts
  • flake-utils
  • git-hooks
  • nix-darwin
  • nix-editor
  • nix2container
  • treefmt-nix

• Opens a separate PR against develop on its own branch to avoid conflicts with the monthly updates

• nix-eval-jobs was the only input with a nixpkgs dependency that did not follow this flake's nixpkgs

• Adding nix-eval-jobs.inputs.nixpkgs.follows = "nixpkgs" ensures it uses the same package set as everything else

• Eliminates a redundant nixpkgs copy from the lock file

Inputs not updated by either workflow

• nixpkgs-oldstable is intentionally pinned to a specific commit for extensions requiring older package versions and must not be updated automatically