fix: fs.file.write idempotent=true (deterministic implies idempotent)

Writing the same content to the same path yields the same result,
matching the deterministic=true + idempotent=true contract.

Author: gfernandf

capabilities/fs.file.write.yaml

@@ -48,7 +48,7 @@ cognitive_hints:
 properties:
   deterministic: true
   side_effects: true
-  idempotent: false
+  idempotent: true
 
 safety:
   trust_level: elevated