fix(ci): preserve GitHub Update branch merge exception

[ruby-automation]

Summary

• Restores the narrowly scoped exception in validate-commit-range for GitHub's auto-generated "Update branch" merge commits, which was lost in EXT-29 (PR ci: harden Conventional Commit validators and Git test isolation #218).
• Expands git log format from %s to %P%x01%cn%x01%ce%x01%s so parent hashes, committer name, and committer email are available alongside the subject in the validation loop.
• A commit is skipped only when all three predicates match simultaneously: exactly 2 parents, committer is GitHub <noreply@github.com>, subject matches ^Merge branch '[^']+' into .+.
• Adds 5 new ExUnit tests covering the happy path and each predicate variation (wrong name, wrong email, single parent, non-matching subject).
• Adds documents/github-update-branch-validation-decision.adoc recording the rationale and alternatives considered.

Test plan

• mix ci passes (405 tests, no lint or format issues)
• New test: GitHub Update branch merge is skipped (exit 0)
• New test: wrong committer name → validated → fails
• New test: wrong committer email → validated → fails
• New test: single-parent commit with matching subject → validated → fails
• New test: two-parent GitHub-committer merge with non-matching subject → validated → fails
• Existing tests unchanged and passing

Closes EXT-35

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