[ai] Add rule about u""_q literal.

AGENTS.md

@@ -156,6 +156,18 @@ QString currentTitle = tr::lng_settings_title(tr::now);
 rpl::producer<QString> nameProducer = GetNameProducer();
 ```
 
+**Use `_q` for QString literals:**
+
+Prefer the project literal `u"..."_q` instead of the verbose `QStringLiteral("...")` macro when creating `QString` values:
+
+```cpp
+// Prefer this:
+auto text = u"Settings"_q;
+
+// Instead of this:
+auto text = QStringLiteral("Settings");
+```
+
 ## API Usage
 
 ### API Schema Files