docs: PostToolUseFailure error field format is undocumented (it's not just "Exit code N")

The hooks reference just says error is "String describing the error that occurred", no example.

What actually arrives (2.1.220, failed Bash command):

"Exit code 1\nnode:internal/modules/cjs/loader:1424\r\n  throw err;\r\n..."

So: "Exit code N", newline, then the command's full output/stack trace (CRLF on Windows). If you're parsing exit codes or hashing error text you need to know this. I shipped a parser based on the short examples and it captured nothing for weeks.

One real example payload in the docs would fix it. Same class of omission as #52607.