Add pkg/build package replacing goreleaser with native build system:
- Project discovery (go.mod, wails.json, package.json, composer.json)
- Go cross-compilation with GOOS/GOARCH, CGO_ENABLED=0, ldflags
- Config loading from .core/build.yaml with sensible defaults
- Archive creation (tar.gz for linux/darwin, zip for windows)
- SHA256 checksum generation with CHECKSUMS.txt
CLI integration via `core build`:
- Auto-detect project type or specify with --type
- Cross-compile with --targets (e.g., linux/amd64,darwin/arm64)
- CI mode with --ci for JSON output
- Archive/checksum flags (--archive, --checksum)
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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