feat(tarko): add runtime settings api in server-next

[cjraft]

Summary

1. add runtime settings api in agent-server-next, update related AgentSession code and server routes
2. support agent mode in omni agent; update system prompt for gui-only scenario

Checklist

• Added or updated necessary tests (Optional).
• Updated documentation to align with changes (Optional).
• Verified no breaking changes, or prepared solutions for any occurring breaking changes (Optional).
• My change does not involve the above items.

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[ulivz]

LGTM