ui: clarify orchestrator mode behavior

[hanzheng1954]

Summary

• Clarify that orchestrator is an advanced conversation mode, not just another tool
• Show a warning when orchestrator is enabled that the main agent delegates execution and will not run shell/file tools directly
• Recommend leaving orchestrator off for normal coding sessions

Testing

• pnpm --dir ui run --silent build
• go test ./server ./claudetool

[cla-bot]

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