feat: start sops edit on the key's line

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

internal/ui/app/repo_test.go

@@ -323,3 +323,22 @@ func TestEditorArgs(t *testing.T) {
 		t.Errorf("no line: %q", got)
 	}
 }
+
+func TestEditorAtLine(t *testing.T) {
+	t.Parallel()
+	for _, c := range []struct {
+		editor string
+		line   int
+		want   string
+		ok     bool
+	}{
+		{"nvim", 4, "nvim +4", true},
+		{"/usr/bin/vim -p", 4, "/usr/bin/vim -p +4", true},
+		{"code --wait", 4, "", false}, // sops adds the file last: no -g file:N
+		{"nvim", 0, "", false},
+	} {
+		if got, ok := editorAtLine(c.editor, c.line); got != c.want || ok != c.ok {
+			t.Errorf("%q at %d: %q %v", c.editor, c.line, got, ok)
+		}
+	}
+}