feat: comprehensive NixOS support improvements

[bcotton]

This commit adds full NixOS support and automates Nix package maintenance:

Static Linux Binaries for NixOS

Changes .goreleaser.yml to create static Linux binaries:

• Set CGO_ENABLED=0 for Linux amd64 and arm64 builds
• Remove cross-compiler dependencies (aarch64-linux-gnu-gcc)
• Simplifies build process while fixing NixOS compatibility

Static binaries work on all Linux distributions including NixOS, Alpine, and musl-based distros without dynamic linker dependencies.

Automated default.nix Version Management

Adds default.nix to the version bump workflow:

• Updates default.nix version field in bump-version.sh (new step 9)
• Adds default.nix to version verification checks
• Prevents version drift (was 5 releases behind: 0.42.0 vs 0.47.0)
• Updates README.md to remove glibc 2.32+ requirement

Automated vendorHash Management

Creates scripts/update-nix-vendorhash.sh to automate vendorHash updates:

• Automatically detects correct hash by triggering Nix build error
• Extracts hash from error message and updates default.nix
• Verifies update with clean build
• Eliminates error-prone manual copy-paste workflow
• Works with local Nix OR Docker (uses nixos/nix image automatically)

Integrates vendorHash check into bump-version.sh:

• Detects when go.mod or go.sum have changed
• Prompts to run vendorHash update script interactively
• Catches synchronization issues at release time

Documentation

Updates AGENTS.md with Nix package maintenance guide:

• Documents when and how to update vendorHash
• Recommends automated script as primary method
• Provides manual and alternative methods as fallback
• Notes Docker fallback for maintainers without Nix

Impact

• NixOS users can now install via standard methods
• Nix package version stays synchronized automatically
• vendorHash updates work without Nix installed (via Docker)
• vendorHash updates are caught during release workflow
• All Linux users benefit from more portable binaries

[bcotton]

Thanks for the detailed review! I've addressed all three issues:

✅ Fixed Issues

1. Undefined log_ functions* - Replaced all log_warning(), log_info(), log_success(), and log_error() calls with the existing echo pattern using ${YELLOW}, ${GREEN}, and ${RED} colors (consistent with the rest of bump-version.sh)

2. Version drift - Updated default.nix version from 0.42.0 to 0.47.0 to match cmd/bd/version.go

3. AGENTS.md architecture - Removed the Nix Package Maintenance section (48 lines) to keep AGENTS.md minimal per beads' dynamic priming design

All changes committed in 88cafd1c and ready for re-review.