Address review: faithful conflict resolution, run-name matching, e2e helpers

- follow_conflict_comment now rewrites only the conflicted region to a
  sentinel line, keeping changes the merge brought in cleanly. The previous
  whole-file `git checkout --ours` dropped any clean incoming change, so the
  test only matched a human resolution by accident of the fixtures.
- The workflow-run waiters match the run-name prefix through " - ", so
  "PR #2" no longer prefix-matches "PR #20"; the head-branch tiebreaker is
  gone.
- edit_and_commit / create_pr replace the stack-setup boilerplate repeated
  across every scenario.
- Add the missing Scenario 3 entry to the top-of-file index.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: Phlogistique