Remove ensure_built_in_actors() calls from ActorRegistry.list() and
ActorRegistry.list_actors() so the read-only 'agents actor list' command
no longer triggers database writes (upsert_actor, set_default_actor).
Root cause: list_actors() unconditionally called ensure_built_in_actors()
which iterated all configured providers and upserted each one — a database
write on every list invocation. This caused migration prompts on fresh
checkouts and violated the read-only contract of the list command.
Includes TDD regression tests from #841 with @tdd_expected_fail removed
per Bug Fix Workflow. Updates three existing test suites (consolidated_actor,
tdd_actor_list_validation, actor_list_empty) that relied on the old
behavior where list_actors() triggered ensure_built_in_actors() — they
now call ensure_built_in_actors() explicitly before listing.
ISSUES CLOSED: #797
brent.edwards