Add flake.nix with computed vendorHash for Go dependencies. The flake
supports all major platforms (x86_64/aarch64 Linux and macOS) and
builds the bd binary from cmd/bd.
Removed unnecessary postInstall section as buildGoModule already names
the binary correctly from subPackages.
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude <noreply@anthropic.com>
perf: Use self input instead of ./. for faster Nix evaluation
Replace src = ./. with src = self in flake.nix to reduce unnecessary
file copying during evaluation. This eliminates the Nix warning about
evaluation performance.
