Clarify mcfly instructions (#64446)

Author: RyanCavanaugh

.github/skills/mcfly/SKILL.md

@@ -4,18 +4,19 @@ description: Only invoke when explicitly instructed to
 ---
 
 You need to "re-stack" your commits to match the following format:
- * A failing test case
+ * Testcase(s), including at least one that demonstrate the problem - this could be a test that directly fails, baselines that show the problem, or even both
  * A commit, or series of commits, that fixes the failing test case
- * (Optional) a follow-up commit that checks in any modified baselines
+ * If needed, a follow-up commit with modified baselines
 
 Since you can't force-push, start with a commit that undoes all your prior changes and gets you back to `main`.
 
 Then add your test case(s) in a single commit.
+When you add testcases that create or modify baselines, commit the (pre-fix) baselines at the same time as the testcases themselves.
 A test case isn't necessarily a new file; it could be a modification to an existing test or a change in the test configuration.
 Ensure that at least one test in the full run fails - you might be operating on a bad PR that didn't actually introduce a failing scenario!
 If zero tests failed, abort and post a message explaining what you observed.
 Remember that baseline tests do not "fail" in the traditional sense; they are used to verify that the output matches the expected baseline.
-You should examine the PR description to determine what the desired passing baseline should look like.
+Ensure that your test demonstrates the problem that the PR is intended to fix.
 
 Then, add your fix.
 You can break this apart into multiple commits if it's more logical to review that way; use your judgment.