Bazel 9 rejects `use_repo_rule` loads of private (underscore-prefixed) symbols, so `bazel mod deps` in MMS fails with:

```
ERROR: @@+bazel_rules_mongo_extension+bazel_rules_mongo//codeowners:codeowners_validator.bzl does not export a repository_rule called _codeowners_validator
```

Changes in `buildscripts/bazel_rules_mongo`:

• `codeowners/codeowners_validator.bzl`: rename `_codeowners_validator` → `codeowners_validator_repository`
• `codeowners/codeowners_binary.bzl`: rename `_codeowners_binary` → `codeowners_binary_repository`
• `README.md`: document bzlmod usage via `use_repo_rule`
• `version.txt`: bump version 0.1.17 → 0.1.18 so the nightly `package_bazel_rules_mongo` task publishes it

Backward compatibility: the `codeowners_validator()` / `codeowners_binary()` WORKSPACE macros and the `*_extension` module extensions are unchanged, so mongo's `WORKSPACE.bazel` needs no update. MMS will need to switch its `use_repo_rule` calls to the new names after 0.1.18 is published.

Testing: `bazel fetch @codeowners_binary//:codeowners @codeowners_validator//:codeowners-validator` succeeds in mongo.