Unexport ulidPattern (extraction shape, not a validator)

After moving ULID validation to oklog/ulid, the ULID regex no longer validates — it only composes CheckpointPattern to extract a candidate ID token from free text (the Entire-Checkpoint trailer). Nothing outside the id package referenced the exported ULIDPattern, and an exported "shape" regex that looks like a validator is a footgun, so unexport it to ulidPattern. Pattern stays exported (run IDs in investigate/provenance use it); CheckpointPattern stays exported (trailer scan).