Pre-release fixes and polish for open source launch

Fixed critical issues identified in code review:
- Fixed invalid Go version (1.25.2 → 1.21) in go.mod
- Fixed unchecked error in import.go JSON unmarshaling
- Fixed unchecked error returns in test cleanup (export_import_test.go, import_collision_test.go)
- Removed duplicate test code in dependencies_test.go via helper function

Added release infrastructure:
- Added 'bd version' command with JSON output support
- Created comprehensive CHANGELOG.md following Keep a Changelog format
- Updated README.md with clear alpha status warnings

All tests passing. Ready for public repository opening.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

cmd/bd/version.go

@@ -0,0 +1,33 @@
+package main
+
+import (
+	"fmt"
+
+	"github.com/spf13/cobra"
+)
+
+const (
+	// Version is the current version of bd
+	Version = "0.9.0"
+	// Build can be set via ldflags at compile time
+	Build = "dev"
+)
+
+var versionCmd = &cobra.Command{
+	Use:   "version",
+	Short: "Print version information",
+	Run: func(cmd *cobra.Command, args []string) {
+		if jsonOutput {
+			outputJSON(map[string]string{
+				"version": Version,
+				"build":   Build,
+			})
+		} else {
+			fmt.Printf("bd version %s (%s)\n", Version, Build)
+		}
+	},
+}
+
+func init() {
+	rootCmd.AddCommand(versionCmd)
+}