Cover conflict resolution matrix in e2e

[Phlogistique]

The conflict-resolution path has several distinct states now: a clean automatic update, an immediate base-branch conflict, an immediate trunk conflict, both conflicts in one run, and a follow-up trunk conflict after the user resolves the base conflict. The existing e2e coverage mostly exercised the trunk-conflict case.

This makes the generated conflict comment easier to test by splitting executable commands into separate bash blocks, then teaches the e2e test to run those blocks directly. The new matrix scenarios assert the exact merge refs printed in the comment and verify that the final PR diffs contain only the intended resolved conflict changes.

Validated with bash -n update-pr-stack.sh, bash -n tests/test_e2e.sh, bash tests/test_update_pr_stack.sh, bash tests/test_rebase_workflow.sh, bash tests/test_mixed_workflows.sh, git diff --check, and shellcheck update-pr-stack.sh tests/test_e2e.sh (existing warnings only). The full GitHub e2e was left to CI because it creates and deletes a real repository.

[github-actions]

⚠️ Automatic update blocked by merge conflicts

I tried to merge origin/push-base-merge-before-conflict-comment into this branch while updating the pull request stack and hit conflicts.

How to resolve

git fetch origin
git switch test-conflict-resolution-matrix
git pull origin test-conflict-resolution-matrix
git merge origin/push-base-merge-before-conflict-comment
# ...
# fix conflicts, for instance with `git mergetool`
# ...
git commit
git push

Once you push, this action will resume and finish updating this pull request.

[Phlogistique]

Suggested change

	for i in "${!CONFLICTS[@]}"; do
	echo '```bash'
	if [[ "$i" -eq 0 ]]; then
	echo "git fetch origin"
	echo "git switch $BRANCH"
	echo "git pull origin $BRANCH"
	fi
	echo "git merge ${CONFLICTS[$i]}"
	echo '```'
	echo
	echo 'Fix the conflicts (for instance with `git mergetool`), then run `git commit` before continuing.'
	echo
	done
	echo "git push"
	echo '```bash'
	echo '```bash'
	echo "git fetch origin"
	echo "git switch $BRANCH"
	echo "git pull origin $BRANCH"
	for i in "${!CONFLICTS[@]}"; do
	echo "git merge ${CONFLICTS[$i]}"
	echo '```'
	echo
	echo 'Fix the conflicts (for instance with `git mergetool`), then run `git commit` before continuing.'
	echo
	echo '```bash'
	done