Adds browser-level coverage for the pull request merge box, which
currently had no tests. Exercises the full merge flow: open the PR,
expand the merge form, submit, and assert the merged state.
Extracted from https://github.com/go-gitea/gitea/pull/36759.
Also updated AGENTS.md with instructions for e2e tests.
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Signed-off-by: silverwind <me@silverwind.io>
Co-authored-by: Giteabot <teabot@gitea.io>
silverwind