fix(newos4): agent read gets its own chat cursor (v4.6.76)

Two readers shared one cursor. The 30 s UI poller printed each reply and advanced
webx11_chat_state.cursor, so the agent's chat_hermes read then asked for 'since=<that>'
and got an empty list — the answer appeared on the terminal and never reached the model.
That is the reported symptom exactly.

chat_hermes read now uses webx11_chat_agent_cursor, starts from 0 on first use so the
whole thread is visible, and never touches the display cursor. Empty reads say why
('still awaiting Hermes' vs 'no new messages since your last read').

Proven: UI poll consumed the reply (printed to terminal), then chat_hermes read still
returned both the agent's question and Hermes's answer.

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