Document that model datetime attributes are already ISO strings

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

docs/CLAUDE.dist.md

@@ -951,6 +951,8 @@ Sessions persist 365 days. Never implement "Remember Me".
 
 **String-Based**: ISO strings, not Carbon. Never use `$casts` with `'date'`/`'datetime'` - blocked by `rsx:check`.
 
+**MODEL ATTRIBUTES ARE ALREADY STRINGS**: `$model->created_at` returns `"2025-12-24T15:30:00.000Z"`, NOT Carbon. Never call `->format()` on datetime attributes. Use `Rsx_Time::format($model->created_at)`.
+
 Pattern recognition:
 - `Rsx_Time::now()`, `Rsx_Time::format()`, `Rsx_Time::relative()`
 - `Rsx_Date::today()`, `Rsx_Date::format()`, `Rsx_Date::is_past()`