fix: per-message agent attribution broken when snapshotting

[scmmishra]

In multi-agent conversations, MessageExtractor.extract_messages stamps every assistant message with the currently active agent's name. After a handoff, the chat contains assistant messages produced by earlier agents, but snapshotting the chat under the final agent re-attributes all of them to that final agent.

That polluted snapshot then leaks into AgentRunner#determine_conversation_agent, which picks the next active agent from the last assistant message's agent_name — so subsequent runs can start with the wrong specialist.

Example of the broken snapshot after a Triage → Billing handoff:

[
  { role: :user, content: "I need help with billing" },
  { role: :assistant, content: "I'll route this.",            agent_name: "Billing" }, # wrong
  { role: :assistant, content: "I can help with your invoice.", agent_name: "Billing" }
]

Fixes #67.

Fix

Attach agent ownership to each assistant message at the moment it is produced, then read that per-message attribution at extraction time instead of inferring it from current_agent.

RubyLLM::Message has no metadata/extension API, so attribution is stored as an SDK-namespaced instance variable (:@agents_authoring_agent) on the message object. Two new module functions on MessageExtractor encapsulate this storage:

• assign_agent_name(message, agent_name) — stamp a message.
• attributed_agent_name_for(message) — read the stamp back.

Runner calls assign_agent_name in two places:

1. After every chat.ask / chat.complete round trip, on the assistant messages appended during that turn (assign_agent_name_to_new_assistant_messages).
2. When restoring messages from context[:conversation_history], preserving any prior agent_name (assign_restored_agent_name).

MessageExtractor.extract_user_or_assistant_message now reads attributed_agent_name_for(msg) first and falls back to current_agent.name only when no per-message attribution exists. This keeps legacy/un-tagged history backwards compatible.

Result

[
  { role: :user, content: "I need help with billing" },
  { role: :assistant, content: "I'll route this.",            agent_name: "Triage" },
  { role: :assistant, content: "I can help with your invoice.", agent_name: "Billing" }
]

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[scmmishra]

@codex please review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Hooray!

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