feat(refinery): implement handleSuccess for merge queue

Implement success handling for the merge queue Engineer:
- Add handleSuccess method that handles successful merge completion
- Update MR body with merge_commit SHA and close_reason
- Close MR with 'merged' reason
- Close source issue with reference to MR ID
- Delete source branch if delete_merged_branches is configured
- Add DeleteRemoteBranch method to git package
- Add git client to Engineer struct
- Add tests for new functionality

Closes gt-3x1.5

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

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

internal/refinery/engineer_test.go

@@ -200,6 +200,9 @@ func TestNewEngineer(t *testing.T) {
 	if e.beads == nil {
 		t.Error("expected beads client to be initialized")
 	}
+	if e.git == nil {
+		t.Error("expected git client to be initialized")
+	}
 	if e.config == nil {
 		t.Error("expected config to be initialized with defaults")
 	}
@@ -207,3 +210,11 @@ func TestNewEngineer(t *testing.T) {
 		t.Error("expected stopCh to be initialized")
 	}
 }
+
+func TestEngineer_DeleteMergedBranchesConfig(t *testing.T) {
+	// Test that DeleteMergedBranches is true by default
+	cfg := DefaultMergeQueueConfig()
+	if !cfg.DeleteMergedBranches {
+		t.Error("expected DeleteMergedBranches to be true by default")
+	}
+}