The legacy pkg/persistence/migrations (DBMigration / dbmXXX) system now
coexists with Alembic but accepts no new migrations — all new schema
changes go through Alembic.
- remove dbm026_llm_model_context_length (superseded by Alembic
0005_add_llm_context_length, which makes the identical change)
- cap required_database_version at 25 (legacy chain dbm001-025 kept
read-only to upgrade pre-existing 3.x DBs to the Alembic baseline)
- add migrations/README.md documenting the freeze
- document the Alembic-only policy and revision-id/idempotency rules in
AGENTS.md
