Add a Nix development environment and submit pgbot to nixpkgs

[nully0x]

Hello here, I came across this project on X and I have been testing it out, I will like to make it available for my OS and workspace and for others alike.

The problem

The existing Go tooling works. This request adds a Nix option on top. It does
not change the current workflow. Today two gaps remain:

• NixOS and Nix package users cannot install pgbot from nixpkgs.
• Nix users cannot start a pinned dev environment with nix develop.

Proposal

Add three small Nix files to the repository root. Then submit the package
recipe to nixpkgs.

The recipe exists and builds. Local validation is complete:

• package.nix builds pgbot versions with buildGoModule. Unit tests pass in
  the sandbox.
• flake.nix provides nix build .#pgbot and nix develop with pinned
  Go 1.25 and golangci-lint.
• Builds verified on x86_64-linux. aarch64-linux verified by cross compilation.

Tasks — repository

• Add package.nix, flake.nix, to the repository root
• Add /result(this is build result from my local test) to .gitignore
• Add a CI job that runs nix build .#pgbot on each push and PR

Tasks — nixpkgs

• Fork NixOS/nixpkgs. Create branch pgbot-init from master.
• Copy the recipe to pkgs/by-name/pg/pgbot/package.nix.
• Add a maintainer entry: username and github ID preferred
• Answer review feedback with new commits.

Follow-up

• On each release: bump the version, refresh the two hashes, and open a
  pgbot: 0.4.X -> 0.4.Y PR.