fix(opencode): answer /plannotator-last feedback with the annotated message's agent (#1614)

OpenCode 1 resolves a prompt with no agent to default_agent and records it as
the session's current agent, so feedback on a custom agent's message was
answered by build. Name the agent that wrote the annotated message on every
delivery path (embedded, CLI bridge, OpenCode 2 native commands); file
annotate uses the session's current agent. Unknown or unaddressable agents
fall back to sending no agent, as before.

Closes #1612