Improve Keystone migration agent with examples and targeted update guidance

.changeset/swift-wolves-migrate.md

@@ -0,0 +1,21 @@
+---
+'@opensaas/stack-cli': minor
+---
+
+Improve KeystoneJS migration agent with side-by-side examples and targeted update guidance
+
+The Keystone migration wizard and agent now produce a targeted migration guide instead of
+regenerating the entire config. Since Keystone and OpenSaaS Stack share the same
+`list()`/field/hook/access API, only imports, the database adapter config, and auth setup
+need to change.
+
+Key improvements:
+
+- The migration agent prompt now includes side-by-side Keystone vs OpenSaaS examples for
+  config structure, imports, access control, hooks, auth, and many-to-many join tables
+- The wizard uses a minimal fast-path for Keystone projects (just 3 questions: db provider,
+  auth, auth methods) instead of the full question flow
+- The generator produces a diff-style migration guide for Keystone showing exactly what to
+  change, rather than regenerating list definitions the user already has
+- Many-to-many join table naming is now surfaced automatically when M2M relations are
+  detected, with `joinTableNaming: 'keystone'` guidance to preserve existing data