fix(check-migration-order): ignore repeatable migrations when finding the newest version [NOJIRA]

[BrianEstrada]

Summary

check-migration-order fails every new migration in a repo whose base branch has a repeatable (R__) migration.

base_max runs every .sql file name through versions(), which only strips a B/V prefix. So
R__monta_service_provider_seed.sql turns into the version R, and sort -V puts that after every numeric
timestamp. Every new V... migration then fails with "does not sort after R".

The fix only counts B/V files followed by a digit when it computes base_max. Repeatable migrations and
callbacks have no version, so they're left out. The README notes this under "Notes".

Found on

monta-app/service-grid#1184. It adds V2026.10.13.10.00__grid_asset_asset_type.sql, and main's newest versioned
migration is V2026.08.13.10.00
(failing job). The
R__ file has been on service-grid's main since 2022, so every service-grid PR that adds a migration fails until this lands.

Test plan

• Ran the unpatched script against service-grid#1184's branch with the action's default naming pattern:
  newest migration on origin/main: R, then the error, exit 1.
• Ran the patched script against the same branch: newest migration on origin/main: 2026.08.13.10.00,
  OK: V2026.10.13.10.00__grid_asset_asset_type.sql sorts after 2026.08.13.10.00, exit 0.
• Ran the patched script against a scratch branch off service-grid main that adds V2026.08.01.10.00__stale.sql.
  It still fails with "does not sort after 2026.08.13.10.00".
• bash -n passes. The naming check and the edited-migration check are unchanged.