refactor(persistence): freeze legacy DB migration chain, drop dbm026

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- remove dbm026_llm_model_context_length (superseded by Alembic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

src/langbot/pkg/utils/constants.py

@@ -2,8 +2,14 @@ import langbot
 
 semantic_version = f'v{langbot.__version__}'
 
-required_database_version = 26
-"""Tag the version of the database schema, used to check if the database needs to be migrated"""
+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