Implement auto-merge functionality for duplicates command

When --auto-merge is used, performMerge now automatically:
1. Closes source issues with "Duplicate of <target>" reason
2. Links each source to target with a "related" dependency

Closes bd-hdt

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

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

cmd/bd/duplicates_test.go

@@ -262,3 +262,107 @@ func TestDuplicatesIntegration(t *testing.T) {
 		t.Errorf("Expected duplicate group to contain 2 'Fix authentication bug' issues, got %d", dupCount)
 	}
 }
+
+func TestPerformMerge(t *testing.T) {
+	tmpDir := t.TempDir()
+	testStore := newTestStore(t, tmpDir+"/.beads/beads.db")
+	ctx := context.Background()
+
+	// Set up global state needed by performMerge
+	oldStore := store
+	oldRootCtx := rootCtx
+	oldActor := actor
+	store = testStore
+	rootCtx = ctx
+	actor = "test-user"
+	defer func() {
+		store = oldStore
+		rootCtx = oldRootCtx
+		actor = oldActor
+	}()
+
+	// Create duplicate issues
+	target := &types.Issue{
+		Title:       "Main issue",
+		Description: "This is the target",
+		Status:      types.StatusOpen,
+		Priority:    1,
+		IssueType:   types.TypeTask,
+	}
+	source1 := &types.Issue{
+		Title:       "Main issue",
+		Description: "This is the target",
+		Status:      types.StatusOpen,
+		Priority:    1,
+		IssueType:   types.TypeTask,
+	}
+	source2 := &types.Issue{
+		Title:       "Main issue",
+		Description: "This is the target",
+		Status:      types.StatusOpen,
+		Priority:    1,
+		IssueType:   types.TypeTask,
+	}
+
+	for _, issue := range []*types.Issue{target, source1, source2} {
+		if err := testStore.CreateIssue(ctx, issue, "test"); err != nil {
+			t.Fatalf("CreateIssue failed: %v", err)
+		}
+	}
+
+	// Perform the merge
+	result := performMerge(target.ID, []string{source1.ID, source2.ID})
+
+	// Verify result structure
+	closedIDs := result["closed"].([]string)
+	linkedIDs := result["linked"].([]string)
+	errors := result["errors"].([]string)
+
+	if len(closedIDs) != 2 {
+		t.Errorf("Expected 2 closed issues, got %d", len(closedIDs))
+	}
+	if len(linkedIDs) != 2 {
+		t.Errorf("Expected 2 linked issues, got %d", len(linkedIDs))
+	}
+	if len(errors) != 0 {
+		t.Errorf("Expected 0 errors, got %d: %v", len(errors), errors)
+	}
+
+	// Verify source issues are closed
+	for _, sourceID := range []string{source1.ID, source2.ID} {
+		issue, err := testStore.GetIssue(ctx, sourceID)
+		if err != nil {
+			t.Fatalf("GetIssue(%s) failed: %v", sourceID, err)
+		}
+		if issue.Status != types.StatusClosed {
+			t.Errorf("Issue %s should be closed, got status %s", sourceID, issue.Status)
+		}
+	}
+
+	// Verify target is still open
+	targetIssue, err := testStore.GetIssue(ctx, target.ID)
+	if err != nil {
+		t.Fatalf("GetIssue(%s) failed: %v", target.ID, err)
+	}
+	if targetIssue.Status != types.StatusOpen {
+		t.Errorf("Target issue should still be open, got status %s", targetIssue.Status)
+	}
+
+	// Verify dependencies were created (GetDependencies returns issues this depends on)
+	for _, sourceID := range []string{source1.ID, source2.ID} {
+		deps, err := testStore.GetDependencies(ctx, sourceID)
+		if err != nil {
+			t.Fatalf("GetDependencies(%s) failed: %v", sourceID, err)
+		}
+		found := false
+		for _, dep := range deps {
+			if dep.ID == target.ID {
+				found = true
+				break
+			}
+		}
+		if !found {
+			t.Errorf("Expected dependency from %s to %s", sourceID, target.ID)
+		}
+	}
+}