fix: unify goose Provider path and fail on pending migrations

Apply BasePath via fs.Sub, route CLI through Provider API, return
ErrPendingMigrations when auto-migrate is disabled, use DownTo for blank.

Co-authored-by: Cursor <cursoragent@cursor.com>

testdata/migrations/00001_init.sql

@@ -0,0 +1,5 @@
+-- +goose Up
+SELECT 1;
+
+-- +goose Down
+SELECT 1;