Create dev18.0.x branch checklist

- [x] Identify base commit for new branch
- [x] According to the [schedule](https://dev.azure.com/devdiv/DevDiv/_wiki/wikis/DevDiv.wiki/10097/Dev17-Release), `main` snapped to `rel/d18.0` on 22 August 2025 at 12PM PDT
- [x] Last [completed Project System insertion PR](https://dev.azure.com/devdiv/DevDiv/_git/VS/pullrequests?_a=completed&assignedTo=6e89082d-fdd2-4442-a310-051df5bdc73c): https://dev.azure.com/devdiv/DevDiv/_git/VS/pullrequest/664018
- [x] Last inserted GitHub PR: https://github.com/dotnet/project-system/pull/9742
- [x] Merge commit of that PR: 0fc481ab7206eecede08b4249d8af9fa350808c4
- [x] Create branch on GitHub at that commit
- [x] https://github.com/dotnet/project-system/tree/dev18.0.x
- [x] ~For branches not matching `dev*` or `feature/*` (usually we skip these)~
- [x] ~Update the [pull-request.yml](https://github.com/dotnet/project-system/blob/main/eng/pipelines/pull-request.yml) (via `pr`) to support PR builds~
- [x] ~Update the [official.yml](https://github.com/dotnet/project-system/blob/main/eng/pipelines/official.yml) (via `trigger`) to have signed builds the new branch~
- [x] Update [version.json](https://github.com/dotnet/project-system/blob/main/version.json) (via `"version"`) to match the version of VS, if needed
- [ ] In new branch: https://github.com/dotnet/project-system/blob/dev18.0.x/version.json (#9752)
- [ ] In `main`: https://github.com/dotnet/project-system/blob/main/version.json (#9751)
- [ ] In the new branch, update the "Build VS Bootstrapper" task in [build-official-release.yml](https://github.com/dotnet/project-system/blob/main/eng/pipelines/templates/build-official-release.yml) and set `channelName` to match the VS insertion branch.
- E.g., for VS `main` the `channelName` is "int.main"; for `rel/d17.8` it would be "int.d17.8"; etc.
- [ ] To run manual insertions, use the [DotNet-Project-System pipeline](https://devdiv.visualstudio.com/DevDiv/_build?definitionId=9675&_a=summary). Enter the GitHub branch as _Branch/tag_, the VS branch as _VS Insertion Branch Name_, and check _Create VS Insertion PR_.
- [ ] Update [Policy Service milestone tracking](https://github.com/dotnet/project-system/blob/main/.github/policies)