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/export_import_test.go

@@ -20,7 +20,11 @@ func TestExportImport(t *testing.T) {
 	if err != nil {
 		t.Fatalf("Failed to create temp dir: %v", err)
 	}
-	defer os.RemoveAll(tmpDir)
+	defer func() {
+		if err := os.RemoveAll(tmpDir); err != nil {
+			t.Logf("Warning: cleanup failed: %v", err)
+		}
+	}()
 
 	dbPath := filepath.Join(tmpDir, "test.db")
 
@@ -219,7 +223,11 @@ func TestExportEmpty(t *testing.T) {
 	if err != nil {
 		t.Fatalf("Failed to create temp dir: %v", err)
 	}
-	defer os.RemoveAll(tmpDir)
+	defer func() {
+		if err := os.RemoveAll(tmpDir); err != nil {
+			t.Logf("Warning: cleanup failed: %v", err)
+		}
+	}()
 
 	dbPath := filepath.Join(tmpDir, "empty.db")
 	store, err := sqlite.New(dbPath)
@@ -263,7 +271,11 @@ func TestRoundTrip(t *testing.T) {
 	if err != nil {
 		t.Fatalf("Failed to create temp dir: %v", err)
 	}
-	defer os.RemoveAll(tmpDir)
+	defer func() {
+		if err := os.RemoveAll(tmpDir); err != nil {
+			t.Logf("Warning: cleanup failed: %v", err)
+		}
+	}()
 
 	dbPath := filepath.Join(tmpDir, "original.db")
 	store, err := sqlite.New(dbPath)