Implement merge validation logic (bd-192)

- Add merge command with --into and --dry-run flags
- Validate target and source issues exist
- Validate no self-merge attempts
- Add comprehensive test coverage
- Capture --id flag feature request as bd-9369

Amp-Thread-ID: https://ampcode.com/threads/T-22945597-9f4f-413b-afde-dcf3099eb2f0
Co-authored-by: Amp <amp@ampcode.com>

cmd/bd/merge_test.go

@@ -0,0 +1,182 @@
+package main
+
+import (
+	"context"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/steveyegge/beads/internal/storage/sqlite"
+	"github.com/steveyegge/beads/internal/types"
+)
+
+func TestValidateMerge(t *testing.T) {
+	tmpDir := t.TempDir()
+	dbFile := filepath.Join(tmpDir, ".beads", "issues.db")
+	if err := os.MkdirAll(filepath.Dir(dbFile), 0755); err != nil {
+		t.Fatalf("Failed to create test directory: %v", err)
+	}
+
+	testStore, err := sqlite.New(dbFile)
+	if err != nil {
+		t.Fatalf("Failed to create test storage: %v", err)
+	}
+	defer testStore.Close()
+
+	store = testStore
+	ctx := context.Background()
+
+	// Create test issues
+	issue1 := &types.Issue{
+		ID:          "bd-1",
+		Title:       "Test issue 1",
+		Description: "Test",
+		Priority:    1,
+		IssueType:   types.TypeTask,
+		Status:      types.StatusOpen,
+	}
+	issue2 := &types.Issue{
+		ID:          "bd-2",
+		Title:       "Test issue 2",
+		Description: "Test",
+		Priority:    1,
+		IssueType:   types.TypeTask,
+		Status:      types.StatusOpen,
+	}
+	issue3 := &types.Issue{
+		ID:          "bd-3",
+		Title:       "Test issue 3",
+		Description: "Test",
+		Priority:    1,
+		IssueType:   types.TypeTask,
+		Status:      types.StatusOpen,
+	}
+
+	if err := testStore.CreateIssue(ctx, issue1, "test"); err != nil {
+		t.Fatalf("Failed to create issue1: %v", err)
+	}
+	if err := testStore.CreateIssue(ctx, issue2, "test"); err != nil {
+		t.Fatalf("Failed to create issue2: %v", err)
+	}
+	if err := testStore.CreateIssue(ctx, issue3, "test"); err != nil {
+		t.Fatalf("Failed to create issue3: %v", err)
+	}
+
+	tests := []struct {
+		name      string
+		targetID  string
+		sourceIDs []string
+		wantErr   bool
+		errMsg    string
+	}{
+		{
+			name:      "valid merge",
+			targetID:  "bd-1",
+			sourceIDs: []string{"bd-2", "bd-3"},
+			wantErr:   false,
+		},
+		{
+			name:      "self-merge error",
+			targetID:  "bd-1",
+			sourceIDs: []string{"bd-1"},
+			wantErr:   true,
+			errMsg:    "cannot merge issue into itself",
+		},
+		{
+			name:      "self-merge in list",
+			targetID:  "bd-1",
+			sourceIDs: []string{"bd-2", "bd-1"},
+			wantErr:   true,
+			errMsg:    "cannot merge issue into itself",
+		},
+		{
+			name:      "nonexistent target",
+			targetID:  "bd-999",
+			sourceIDs: []string{"bd-1"},
+			wantErr:   true,
+			errMsg:    "target issue not found",
+		},
+		{
+			name:      "nonexistent source",
+			targetID:  "bd-1",
+			sourceIDs: []string{"bd-999"},
+			wantErr:   true,
+			errMsg:    "source issue not found",
+		},
+		{
+			name:      "multiple sources valid",
+			targetID:  "bd-1",
+			sourceIDs: []string{"bd-2"},
+			wantErr:   false,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := validateMerge(tt.targetID, tt.sourceIDs)
+			if tt.wantErr {
+				if err == nil {
+					t.Errorf("validateMerge() expected error, got nil")
+				} else if tt.errMsg != "" && !contains(err.Error(), tt.errMsg) {
+					t.Errorf("validateMerge() error = %v, want error containing %v", err, tt.errMsg)
+				}
+			} else {
+				if err != nil {
+					t.Errorf("validateMerge() unexpected error: %v", err)
+				}
+			}
+		})
+	}
+}
+
+func TestValidateMergeMultipleSelfReferences(t *testing.T) {
+	tmpDir := t.TempDir()
+	dbFile := filepath.Join(tmpDir, ".beads", "issues.db")
+	if err := os.MkdirAll(filepath.Dir(dbFile), 0755); err != nil {
+		t.Fatalf("Failed to create test directory: %v", err)
+	}
+
+	testStore, err := sqlite.New(dbFile)
+	if err != nil {
+		t.Fatalf("Failed to create test storage: %v", err)
+	}
+	defer testStore.Close()
+
+	store = testStore
+	ctx := context.Background()
+
+	issue1 := &types.Issue{
+		ID:          "bd-10",
+		Title:       "Test issue 10",
+		Description: "Test",
+		Priority:    1,
+		IssueType:   types.TypeTask,
+		Status:      types.StatusOpen,
+	}
+
+	if err := testStore.CreateIssue(ctx, issue1, "test"); err != nil {
+		t.Fatalf("Failed to create issue: %v", err)
+	}
+
+	// Test merging multiple instances of same ID (should catch first one)
+	err = validateMerge("bd-10", []string{"bd-10", "bd-10"})
+	if err == nil {
+		t.Error("validateMerge() expected error for duplicate self-merge, got nil")
+	}
+	if !contains(err.Error(), "cannot merge issue into itself") {
+		t.Errorf("validateMerge() error = %v, want error containing 'cannot merge issue into itself'", err)
+	}
+}
+
+func contains(s, substr string) bool {
+	return len(s) >= len(substr) && (s == substr || len(s) > len(substr) && containsSubstring(s, substr))
+}
+
+func containsSubstring(s, substr string) bool {
+	for i := 0; i <= len(s)-len(substr); i++ {
+		if s[i:i+len(substr)] == substr {
+			return true
+		}
+	}
+	return false
+}