#21646 ci(cmux-next): run the split guard's classifier from the base

cmux-next split guard / split-plan advisory fails on any pull request whose branch predates #21579: the job checks out the PR head and runs scripts/cmux-next/check-pr-split.py from it, and older heads have no copy (python3: can't open file '…/scripts/cmux-next/check-pr-split.py', for example run 39129034431). The step now reads the classifier from the base commit, which every PR into feat-cmux-next has since #21579. That also keeps a PR from grading itself with its own edits to the guard.

Testing

• actionlint .github/workflows/cmux-next-split-guard.yml: passed.
• The new step's commands under bash in a feat-cmux-next checkout (base HEAD~3, head HEAD): emitted SPLIT_GUARD_JSON and the advisory warning, exit 0.
• This PR's own split-plan advisory run exercises the changed step.

Changelog

none

🤖 Generated with Claude Code

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Summary by cubic

Fixes the cmux-next split guard workflow failing on pull requests whose head branched before #21579, since those heads have no scripts/cmux-next/check-pr-split.py copy.

The classifier is now extracted from the base commit instead of run from the PR head, which also stops a pull request from grading itself with its own edits to the guard.