check for large binaries in CI (#14382)

Prevent binaries >500KB from being committed. And maintain an allowlist
if we need to bypass on a case-by-case basis.

I checked the currently tracked binary-like assets in the repo. There
are only 5 obvious committed binaries by extension/MIME type:
- `.github/codex-cli-splash.png`: `838,131` bytes, about `818 KiB`
- `codex-rs/vendor/bubblewrap/bubblewrap.jpg`: `40,239` bytes, about `39
KiB`
-
`codex-rs/skills/src/assets/samples/skill-creator/assets/skill-creator.png`:
`1,563` bytes
- `codex-rs/skills/src/assets/samples/openai-docs/assets/openai.png`:
`1,429` bytes
-
`codex-rs/skills/src/assets/samples/skill-installer/assets/skill-installer.png`:
`1,086` bytes

So `500 KB` looks like a good default for this repo. It would only trip
on one existing intentional asset, which keeps the allowlist small and
the policy easy to understand.

Here's a smoke-test from a throwaway branch that tries to commit a large
binary:
https://github.com/openai/codex/actions/runs/22971558828/job/66689330435?pr=14383