Export persona.DiscoverAgentsMD and add agent.Config.AgentsMD so project
and global AGENTS.md rules actually reach the model. Previously
buildInitialMessages always called AssembleSystemPrompt with an empty
string, so no AGENTS.md content was ever injected despite the discovery
logic already existing.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>