docs: check for behavior changes against the latest release in the review skill

[andygrove]

Which issue does this PR close?

No issue. This is a small update to the review skill.

Rationale for this change

Users upgrade from a release, not from main. The review skill currently compares a PR only against main, so it can miss a PR that, on its own or together with unreleased changes already on main, changes behavior users had in the latest release. Those are the regressions that show up after an upgrade.

What changes are included in this PR?

review-comet-pr gains a "Behavior change against the latest release" check under the checks that apply to every PR. It tells the reviewer to:

• find the highest branch-X.Y and diff the PR head against it for the touched files
• look for changes in results, errors, native vs fallback decisions, config defaults and support levels, and performance
• use the PR's new tests as a probe, running them against the release branch when the answer is not obvious
• classify each change as intended (documented, and considered for backport) or unintended (a regression that falls under the existing request-changes rule)

The output format gets a "Behavior vs Release" section, and the request-changes case now names unintended behavior changes against the release explicitly.

How are these changes tested?

Docs only. Ran prettier on the file and checked that the git ls-remote command picks branch-1.1 today.

[andygrove]

Combined into #6516, which has these changes plus the review-skill lessons from the 1.1.0 regression audit (#6399).