fix: properly inject version and commit hash into build
- Add ldflags in flake.nix to set version and commit hash at build time
- Pass version and commit to fang.Execute() via WithVersion and WithCommit options
- Fixes version output showing 'unknown (built from source)'
- Version now correctly displays as 'herald version 0.1.1 (hash)'
💘 Generated with Crush
Assisted-by: Copilot: Claude Sonnet 4.5 via Crush <crush@charm.land>