docs: gh is required for the release-branch pull requests; ask the user to install it

AGENTS.md

@@ -114,8 +114,10 @@ gamescope and the Plasma desktop. Primary target: the Valve Steam Machine
     - Up to the release branch, the agent manages it: create the
       `feature/`/`bugfix/` branches, commit, push, and merge them into the
       release branch itself once tested (with `gh`: `gh pr create --base
-      release/X.Y.Z` + `gh pr merge --merge`, or a plain `git merge --no-ff`;
-      no review needed). Only the
+      release/X.Y.Z` + `gh pr merge --merge`; no review needed). Without
+      `gh` (`command -v gh`), ask the user to install it (Ubuntu:
+      `sudo apt install gh`, then `gh auth login`) instead of working around
+      it. Only the
       release branch's pull request into `main` is the user's: never commit
       or merge to `main`, the user merges that one.
       Keep a feature/bugfix branch up to date by merging (or rebasing on)