test(e2e): add pull request merge box test, update AGENTS.md (#38576)

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>

AGENTS.md

@@ -12,7 +12,8 @@
 - Preserve existing code comments, do not remove or rewrite comments that are still relevant
 - Keep comments short, prefer same-line, explain why, never narrate code
 - Prefer unit tests over integration tests when logic is testable in isolation
-- Aim for sub-2s local runtime for integration and e2e tests
+- Aim for sub-2s local runtime per integration test and sub-4s per e2e test
+- Prefer using semantic selectors in e2e tests
 - In TypeScript, use `!` (non-null assertion) instead of `?.`/`??` when a value is known to always exist
 - For CSS layout, prefer `flex-*` helpers over per-child `tw-ml-*` / `tw-mr-*` margins; fall back to `tw-*` utilities when specificity requires `!important`
 - Include authorship attribution in issue and pull request comments