test(merge): add test for both sides adding same dependency

Ensures no duplicates when left and right independently add the same
dependency (not in base). Left version is preferred.

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

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

internal/merge/merge_test.go

@@ -180,6 +180,19 @@ func TestMergeDependencies(t *testing.T) {
 				{IssueID: "bd-1", DependsOnID: "bd-3", Type: "related", CreatedAt: "2024-01-01T00:00:00Z"},
 			},
 		},
+		{
+			name: "both add same dep (not in base) - no duplicates",
+			base: []Dependency{},
+			left: []Dependency{
+				{IssueID: "bd-1", DependsOnID: "bd-2", Type: "blocks", CreatedAt: "2024-01-01T00:00:00Z"},
+			},
+			right: []Dependency{
+				{IssueID: "bd-1", DependsOnID: "bd-2", Type: "blocks", CreatedAt: "2024-01-02T00:00:00Z"},
+			},
+			expected: []Dependency{
+				{IssueID: "bd-1", DependsOnID: "bd-2", Type: "blocks", CreatedAt: "2024-01-01T00:00:00Z"}, // Left preferred
+			},
+		},
 		{
 			name: "left removes dep from base - REMOVAL WINS",
 			base: []Dependency{