Fix empty chat history when switching agents

[rogerchappel]

Summary

• load agent thread history from the OpenClaw gateway session instead of the app DB when switching agents
• keep each agent thread scoped to its own session key so superior/child conversations appear in the correct thread
• scope chat session reuse by company when persisting new messages

Testing

• pnpm build
• pre-push hooks: pnpm typecheck, pnpm build