feat: add release build script for multi-platform binaries

- Add scripts/release.sh for automated release builds
- Creates tar.gz packages for Linux and macOS
- Generates SHA256 checksums
- Add 'make release' target
- Add releases/ to .gitignore

Usage:
  make release  # Build and package for all platforms

Output: releases/v0.2.5/*.tar.gz + SHA256SUMS.txt

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Makefile

@@ -36,6 +36,10 @@ build-macos:
 # Build all platforms
 build-all: build-release build-linux build-macos
 
+# Create release packages for all platforms
+release:
+	@./scripts/release.sh
+
 # Show version
 version:
 	@echo "Version: $(VERSION)"
@@ -74,6 +78,7 @@ help:
 	@echo "  build-linux    Cross-compile for Linux"
 	@echo "  build-macos    Cross-compile for macOS (Intel + Apple Silicon)"
 	@echo "  build-all      Build for all platforms"
+	@echo "  release        Create release packages for all platforms"
 	@echo "  version        Show current version"
 	@echo "  clean          Remove build artifacts"
 	@echo "  test           Run tests"