docs: require audit skill to file issues and add Spark 4.1.1 to version list

[andygrove]

Which issue does this PR close?

Closes #.

Rationale for this change

Two improvements to the audit-comet-expression skill surfaced by the string-expressions audit in #4461.

1. The skill previously permitted leaving semantics-decision findings as prose recommendations in the PR description, on the assumption the reviewer would pick them up. In practice that note dies with the PR. Several higher-risk findings from the string audit (CometCaseConversionBase compat gating, StringRepeat negative-count divergence, translate grapheme semantics, bit_length/octet_length BinaryType native error, decode legacy flags) had to be filed retroactively as [Bug] repeat throws on negative count where Spark returns empty string #4462-[Bug] CometCaseConversionBase gates compat inside convert() instead of getSupportLevel #4467 because the previous skill version did not enforce filing an issue.
2. The skill's Spark version list still only covered 3.4.3, 3.5.8, and 4.0.1. Spark 4.1.1 is now a tracked release in the project and should be diffed against alongside the others.

What changes are included in this PR?

• Tighten Step 6 and Step 7 so that every high-priority finding either becomes an inline fix + test, or a filed GitHub issue + ignored regression test, before the audit PR is opened. Add a dedicated "Findings that need follow-up" subsection spelling out the workflow (search, file with correctness / documentation label, cross-reference from the support-doc sub-bullet and the PR description).
• Add Spark v4.1.1 to every for tag in ... loop in Step 1 / Step 2, add a 4.0.1 → 4.1.1 row to the cross-version diff list, and update the descriptions, Step 8 sub-bullet template, and Step 5 output-format section to enumerate four Spark versions.

How are these changes tested?

Skill-only documentation change. The next per-category audit will exercise both new behaviours: it will clone Spark 4.1.1 alongside the existing versions, and any high-priority finding it cannot fix inline will be filed as a tracking issue rather than left as prose.