fix: add warning about identical oldString and newString in edit tool

packages/opencode/src/tool/edit.txt

@@ -7,4 +7,5 @@ Usage:
 - Only use emojis if the user explicitly requests it. Avoid adding emojis to files unless asked.
 - The edit will FAIL if `oldString` is not found in the file with an error "oldString not found in content".
 - The edit will FAIL if `oldString` is found multiple times in the file with an error "oldString found multiple times and requires more code context to uniquely identify the intended match". Either provide a larger string with more surrounding context to make it unique or use `replaceAll` to change every instance of `oldString`. 
+- The edit will FAIL if `oldString` and `newString` are identical strings: you MUST NEVER pass identical arguments for the `oldString` and `newString` arguments!
 - Use `replaceAll` for replacing and renaming strings across the file. This parameter is useful if you want to rename a variable for instance.