feat(migrate): configure embedded Goose providers (#14)

Add embedded SQL and provider-local Go migrations with slog logging and
Postgres session locking, returning Goose's Provider directly.

Cover timestamp and sequential versions, failure recovery, and concurrent
startup with SQLite and Postgres tests. Reuse page's Postgres CI service.
Document authoring conventions and the Go template compatibility gate.

Closes #7
Refs #9

README.md

@@ -2,6 +2,11 @@
 
 Shared Go building blocks. Includes `docs`, a checker for architecture decision records (ADRs) and implementation plans; `page`, Bob keyset pagination; and `lifecycle`, a service and worker coordinator.
 
+## migrate
+
+[`migrate`](migrate/README.md) configures Goose providers for embedded SQL and Go
+migrations, with structured logging and Postgres startup locking.
+
 ## docs
 
 Install: