revert(skills): leave the field-types rule line to its own skills-lane card

The '(no currency)' comment goes false when the currency result lands; it moves
to a skills card blocked on this one, so this PR touches no governed surface.

Claude-Session: https://claude.ai/code/session_016njDy8ozy9B9Ns5Y8kAWEK
Co-authored-by: Claude <noreply@anthropic.com>

Author: claude

skills/objectstack-data/rules/field-types.md

@@ -250,7 +250,7 @@ import { F } from '@objectstack/spec';
 const total = {
   type: 'formula',
   expression: F`record.amount * record.tax_rate`,  // CEL — `record.` prefixes required
-  returnType: 'number',   // 'number' | 'text' | 'boolean' | 'date' | 'currency' (+ currencyConfig)
+  returnType: 'number',   // 'number' | 'text' | 'boolean' | 'date' (no 'currency')
 };
 ```