Add the required status checks to the `main` ruleset once #248 merges

Follow-up to #213 / #248. Once #248 is merged, the workflows report these checks on every pull request (skipping via `detect-changes` when nothing relevant changed), so they can be made required without blocking non-C++ pull requests.

In *Settings → Rules → Rulesets → main → Require status checks to pass*, add:

- `GCC trunk Release`
- `GCC trunk Debug`
- `GCC-16 Release`
- `GCC-16 Debug`
- `asan`
- `tsan`
- `uv-lock`

`pre-commit` is already required. Leave the two `detect-changes` jobs out.

Do not enable these before #248 merges: with the current `paths:` filters a required check that never reports leaves every non-C++ pull request stuck on "Expected — Waiting for status to be reported".

#213 can be closed once the ruleset is updated.