make the E2E test more explicit so agents decide not to do just 2 instead of 3 commits

[Soph]

Turns out Copilot CLI (with gpt-4.1) sometimes stuck to making 3 commits and sometimes decided 2 are fine too. This should make our ask more explicit.

[cursor]

PR Summary

Low Risk
Low risk: only changes an E2E test prompt string, affecting test determinism rather than production behavior.

Overview
Updates TestRapidSequentialCommits to use a stricter, step-by-step prompt that explicitly instructs running git add and git commit (with specific messages) after creating each of the three markdown files.

This is intended to reduce flaky agent behavior where fewer than three commits are produced, while keeping the test assertions the same (3 new commits, each with a checkpoint trailer).

^{Written by Cursor Bugbot for commit 8eec694. Configure here.}

[Copilot]

Pull request overview

This PR updates an E2E edge-case prompt to be more deterministic so agents reliably create three separate commits (one per file) rather than occasionally collapsing the work into fewer commits.

Changes:

• Replaced a high-level instruction (“commit each file separately”) with an explicit ordered 3-step sequence.
• Included concrete git add … && git commit -m … commands for each file to reduce agent ambiguity.