Add Nix flake and dev shell for building gh with PR review helpers

[rowan-stein]

Add Nix flake and dev shell for building gh with PR review helpers

Goal
Provide Nix configuration (flake) to build and install a gh binary that includes our new gh pr review commands, plus a dev shell for local development.

Scope

• Add flake.nix and (if needed) flake.lock for reproducible builds.
• Outputs:
  • packages for: x86_64-linux, aarch64-linux, x86_64-darwin, aarch64-darwin
  • defaultPackage for the host platform
  • devShells.default with Go toolchain, golangci-lint, and common tooling
  • apps.gh so nix run .#gh -- <args> works
• Build from branch contrib/pr-review-commands by default (or the current checkout), with proper version stamping (e.g., GH_VERSION pulled from git tag or fallback pre-release version).
• Include an overlay to make gh available via nix profile install for users who prefer overlays.

Acceptance Criteria

• nix build .#gh produces a working gh binary exposing:
  • gh pr see-comments
  • gh pr reply-comment
  • gh pr review pending {open,add,submit}
• nix develop provides a shell with Go toolchain (pinned), golangci-lint, and necessary tools to build/test.
• nix run .#gh -- pr --help works on x86_64-linux.
• README updates: a short section with Nix build/install/run examples.
• Open a PR on the fork (do not merge) with the flake and README changes.

Notes

• Keep this PR separate from the upstream contribution; it’s purely packaging for our fork.
• Prefer flake-parts or standard flake layout; ensure CI can build nix build if we enable it later.