feat(agent-runner): enforce 4.x host-owned execution

src/langbot/pkg/utils/constants.py

@@ -2,15 +2,6 @@ import langbot
 
 semantic_version = f'v{langbot.__version__}'
 
-required_database_version = 25
-"""Tag the version of the legacy (3.x) database schema migration chain.
-
-Frozen at 25: the legacy ``pkg/persistence/migrations`` system (DBMigration /
-dbmXXX_*.py) is deprecated and no longer accepts new migrations. All schema
-changes from here on are managed by Alembic (see
-``pkg/persistence/alembic/versions``). This value only gates the one-time
-upgrade of pre-existing 3.x databases up to the Alembic baseline."""
-
 debug_mode = False
 
 edition = 'community'