docs: remove blanket ban on unsigned integers (#7957)

Drop the AGENTS.md rule that forbids unsigned ints. The blanket guidance
causes unnecessary complexity in cases where values are naturally
unsigned, leading to extra clamping/conversion code instead of using
checked or saturating arithmetic where needed.

AGENTS.md

@@ -11,7 +11,6 @@ In the codex-rs folder where the rust code lives:
 - Always collapse if statements per https://rust-lang.github.io/rust-clippy/master/index.html#collapsible_if
 - Always inline format! args when possible per https://rust-lang.github.io/rust-clippy/master/index.html#uninlined_format_args
 - Use method references over closures when possible per https://rust-lang.github.io/rust-clippy/master/index.html#redundant_closure_for_method_calls
-- Do not use unsigned integer even if the number cannot be negative.
 - When writing tests, prefer comparing the equality of entire objects over fields one by one.
 - When making a change that adds or changes an API, ensure that the documentation in the `docs/` folder is up to date if applicable.