workflows/incomplete-prs: allow deleting PR template comments

[gibfahn]

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• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them? Performance claims (e.g. "this is faster") must include Hyperfine benchmarks.
• Have you written new tests (excluding integration tests) for your changes? Here's an example.
• Have you successfully run brew lgtm (style, typechecking and tests) with your changes locally?

Unsure how to test this. Will the changed check run on this PR?

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• AI was used to generate or assist with generating this PR.

AI did the initial investigation, I made the changes.

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The goal is to ensure people include the key lines from the template,
while not requiring them to exactly keep the text verbatim. In my
PR I accidentally changed too many lines, which caused my PR to
be auto-closed, despite me including all the checkboxes.

This makes the following changes:

1. Don't consider --- markdown ruler lines as required
2. Don't consider <!-- HTML comments --> as required
3. Use <!-- HTML --> comments for the AI note as well as for the other
   ones.

Refs: #22249 (comment)

Full AI investigation:

The check works by normalizing lines (stripping, replacing any [ ]/[x]/[X] with [ ]) and checking that ≥75% of the template's unique non-empty lines appear in the PR body.

The template has 9 unique non-empty lines after normalization:

1. -----
2. <!-- Do not tick a checkbox if you haven't performed its action... -->
3. <!-- Use [x] to mark item done... -->
4–8. The five main checkboxes
4. - [ ] AI was used to generate or assist with generating this PR. *Please specify below how you used AI to help you, and what steps you have taken to manually verify the changes*.
Non-maintainers may only have one AI-assisted/generated PR open at a time.

Your PR body matches 6 of those 9:

- Misses the 2 HTML comments — you stripped them (as most authors do)
- Misses the AI checkbox line — you replaced the full template text with AI was used to generate or assist with generating this PR -> Did the design myself, used AI to write the Ruby., which
 no longer matches the template text

6/9 = 66.7%, which is below the 75% threshold.

If you'd only dropped the HTML comments (8/9 = 88.9%) or only modified the AI checkbox (8/9 = 88.9%), each would pass on its own. Both together pushes it under.

Fix: Keep the AI checkbox text verbatim as it appears in the template, and put your note on a separate line below it (which is exactly what "Please specify below" intends):

- [x] AI was used to generate or assist with generating this PR. *Please specify below how you used AI to help you, and what steps you have taken to manually verify the changes*.
Non-maintainers may only have one AI-assisted/generated PR open at a time.

[MikeMcQuaid]

Thanks @gibfahn, a few comments but seems like a good idea.

[MikeMcQuaid]

Thanks!