convey/chat: rebuild active talents on hydrate from chat stream
When convey restarts between talent_spawned and the matching
finish/error, walk today's chat events to repopulate _active_talents
from spawned-without-terminal entries. Late cortex events for those
use_ids resume the parent chat instead of falling through as
unrouteable.
Co-Authored-By: OpenAI Codex <codex@openai.com>