fix: delegate under the speaking guest's identity and let it reach the host

When a user addresses a teammate by name, routing overwrites the turn's
agent_id with the conversation host, so agent_delegate was running under the
host's identity and rejecting the guest handing work to the host as
"delegate to self". Resolve the delegation source from speaker_agent_id
(falling back to agent_id for nested turns) and include the host in the
delegable roster so it matches the prompt roster [host, *members].

Co-authored-by: cowagent <cow@cowagent.ai>