`dorny/paths-filter` defaults to OR semantics across the pattern array,
so the leading `**` matched every file and the `!...` excludes were
no-ops. The `code` filter has been returning `true` for every PR since
#3615.

Split into two filter steps: `code` moves into its own step with
`predicate-quantifier: every` so excludes actually subtract. The two
re-include workflow files become a separate `typecheck_self` filter that
the `typecheck` job ORs into its `if:`.

Side effect: workflow-file-only PRs that don't touch `pr_checks.yml` or
`typecheck.yml` no longer trigger typecheck. Previously they did because
the filter was broken-true.