Contract 0010: drop the legacy outbox (empty; unread since Billing#90, this release's N-1) (#91)

ddiff --git a/migrations/0010_drop_legacy_outbox.sql b/migrations/0010_drop_legacy_outbox.sql
nnew file mode 100644
iindex 0000000..1b009ea
---- /dev/null
++++ b/migrations/0010_drop_legacy_outbox.sql
@@@ -0,0 +1,5 @@
++-- phase: contract
++-- after: 0006
++-- The SDK outbox (service_outbox, migration 0006, 2026-10-10) has carried every Billing event since. The legacy outbox
++-- was empty, and since Billing#90 (deployed 2026-10-23, this release's N-1) no code reads it.
++DROP TABLE IF EXISTS outbox;