docs(skill): make the PR checkbox rule a hard final gate

[priya-sundaram-dev]

Describe your change:

As requested by @cclauss on #15233, this makes the "tick at least one checkbox" rule impossible to miss.

The requirement was previously buried in a prose paragraph in the middle of the skill, so PR bodies generated straight from the template kept being submitted with every box empty — and algorithms-keeper auto-closes those before a human reads them, so genuinely good changes get closed for a formatting reason (exactly what happened to #15233).

Changes:

• Strengthen the inline warning and spell out that it applies to every PR — including CI, docs, and tooling changes that aren't algorithms — tick the boxes that genuinely apply so the body is never all-empty.
• Add a ## Before you click "Create pull request" final gate at the end of the skill with an explicit re-read the rendered body verify step.

Docs-only change to a skill file; no code, no dependencies.

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Add or change doctests? — Note: Please avoid changing both code and tests in a single pull request.
• Documentation change?

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing.
• All new algorithms include at least one URL that points to Wikipedia or another similar explanation.
• If this pull request resolves one or more open issues then the description above includes the issue number(s) with a closing keyword: "Fixes #ISSUE-NUMBER".

[cclauss]

Please text wrap this lone line...

rumdl check..............................................................Failed

• hook id: rumdl
• exit code: 1

.github/skills/new-pull-request/SKILL.md:22:351: [MD013] Line length 541 exceeds 350 characters

Issues: Found 1 issues in 1/34 files (11ms)

[priya-sundaram-dev]

Done — wrapped that paragraph so the longest line is now 88 chars, well under the MD013 350-char limit. rumdl passes locally. Pushed as f05e240.