Needed explicit prompting for push-to-pull-request-branch in multi-repo checkout

[dsyme]

In a workflow I'm working on that checks out multiple repos I found a much more explicit prompt was needed to set the coding agent up for success in using push_to_pull_request_branch tool from one of the repos:

The checkouts are:

checkout:
  - fetch-depth: 0
    path: ./github-automation
  - repository: github/github
    token: ${{ ... }}
    fetch: ["master", "dsyme/n-plus-1/*"]
    fetch-depth: 20
    path: ./github
    sparse-checkout: |
      ...various directories...

and the explicit prompting I had to add was this:

**How `push_to_pull_request_branch` works with this layout:**

The `push_to_pull_request_branch` tool operates on the `github/github` checkout at `$GITHUB_WORKSPACE/github/`. To push fixes to an existing PR branch:

1. `cd $GITHUB_WORKSPACE/github`
2. `git checkout <pr-branch-name>` — the PR branches are fetched (see checkout config) and available as `origin/<branch>`. With fetch-depth=20 you can check them out locally.
3. Make your edits to the files on disk.
4. `git add <files> && git commit -m "<message>"`
5. Call `push_to_pull_request_branch` with `pull_request_number` and `branch` set to the PR's branch name.

**Critical:** The tool finds the git repo by scanning `$GITHUB_WORKSPACE` subdirectories. It will locate `$GITHUB_WORKSPACE/github/` as the `github/github` repo and find your branch there. You do NOT need to be in the workspace root. Just ensure the branch exists in `$GITHUB_WORKSPACE/github/` with committed changes.

**Do NOT** attempt to `git push` manually, rewrite remotes, or use `gh api` for pushing — the tool handles authentication and signing.

Some kind of sensible generic prompting along these lines should be emitted automatically by GH-AW when push-to-pull-request-branch safe output is used in multi-repo checkout: situations. Added notes should be added for sparseness, shallow depth etc. depending on the features used.