Retrofits the ON DELETE pattern learned after v1 shipped (CASCADE for
tenant-scoped children, SET NULL for historical references, RESTRICT where
the domain type is non-Option) onto tables from 0001-0006 and 0026. Two
deliberate exceptions kept as NO ACTION: audit_log.workspace_id (audit is
append-only and must outlive workspace deletes) and thread_messages.from_agent
(history stays attributable via agents.deleted_at). Two stay NOT NULL as
RESTRICT (agents.managed_by, installed_skills.installed_by) because the
Rust domain type is UserId, not Option<UserId>.
Also includes the previously-orphan .sqlx cache for the usage_events query
in cm-runtime/tests/run_loop.rs, which needed re-recording after the FK
changes touched the metadata.
