Add a rule against unrequested edits to adjacent code #640

I have ponytail enabled globally at full. The rules are all about not adding: no unrequested
abstractions, no boilerplate, fewest files, shortest working diff. None of that stops an agent
from editing code it was never asked to touch on the way past. What I see:

1. Reformatting a file it only opened to read.
2. Rewriting or deleting a comment it did not like.
3. "Improving" adjacent code in the same function, unrelated to the task.
4. Deleting pre-existing dead code it happened to notice.

A diff can be short and still be wrong in this way, and "deletion over addition" arguably
encourages #4.

Suggested addition to the Rules section:

Surgical changes. Touch only what you were asked to touch. Don't "improve" adjacent code,
comments, or formatting. Match existing style, even if you'd do it differently. Remove
imports/variables/functions that YOUR change orphaned, nothing else. Notice unrelated dead
code? Mention it, don't delete it. Test: every changed line traces directly to what was asked
for.