Implement hash based edit tool

What variant of Codex are you using?

cli/core

What feature would you like to see?

Implement hash based edit tool described in https://blog.can.ac/2026/02/12/the-harness-problem/

tl;dr

Now bear with me here. What if, when the model reads a file, or greps for something, every line comes back tagged with a 2-3 character content hash: `` 1:a3|function hello() { 2:f1| return "world"; 3:0e|} `` When the model edits, it references those tags — “replace line 2:f1, replace range 1:a3 through 3:0e, insert after 3:0e.” If the file changed since the last read, the hashes (optimistically) won’t match and the edit is rejected before anything gets corrupted. If they can recall a pseudo-random tag, chances are, they know what they’re editing. The model then wouldn’t need to reproduce old content, or god forbid whitespace, to demonstrate a trusted “anchor” to express its changes off of.

Additional information

_No response_