feature-request: add `prTitle` option to `config.storage.kind = github` for computing PR title

Current

Currently, when you create a PR from Keystatic the PR title is computed by keytatic (or by github).
There is no options to customize it.

Request Proposal

Adding a new prTitle option when config.storage.kind = github.
The new option can be a simple string, but i think using a function is more ready for future additions.

// in keystatic config

config({
  storage: {
    kind: 'github',
    prTitle: (options) => {

      /*
      options: {
        branchName: 'XXXXXXXX',
      }
      */

      return `feat: publish content` + options.branchName;
    }
  }
})

Start by tracing the GitHub storage configuration and the PR-creation path described in the request. Check how the current title is computed and where a configurable prTitle value would be applied; done means GitHub-backed PRs use the configured title behavior while existing configurations continue to work.