[codex] fix dependabot actions cooldown config (#414)

## What changed

Remove the unsupported `cooldown.semver-minor-days` and
`cooldown.semver-patch-days` settings from the `github-actions`
Dependabot ecosystem.

The Bun ecosystem keeps its cooldown settings, and GitHub Actions
updates keep the weekly Tuesday schedule, PR limit, and minor/patch
grouping.

## Why

Dependabot rejects semver-specific cooldown properties for the
`github-actions` ecosystem with:

```
The property `#/updates/0/cooldown/semver-minor-days` is not supported for the package ecosystem `github-actions`.
The property `#/updates/0/cooldown/semver-patch-days` is not supported for the package ecosystem `github-actions`.
```

Removing those keys restores a valid Dependabot config while preserving
the grouped weekly update policy.

## Validation

- Parsed `.github/dependabot.yml` successfully as YAML