The cluster the agent is deciding is locked, and the Review page says it is working

[WaylandYang]

Follow-up to #457 (record 0025's "What a reader sees" gains a paragraph).

What this adds

• A lock on the cluster in flight. When the govern job takes a cluster it marks those pairs stage = adjudicating; decide_review refuses such a pair with a 409 ("The agent is deciding this pair right now; wait for its proposal or turn governance off") while the base's switch is on and its job is running. All three conditions are needed, so a stale mark left by a crashed job, or by the old adjudicator when the switch is off, locks nothing; the job clears every mark when it starts and when it ends, whatever way it ends.
• The Review page shows it. A locked duplicate card shows "The agent is deciding this pair" with a pulsing dot and its Merge / Keep buttons and checkbox off; "Select this page" skips locked pairs. The Agent item in the rail carries a pulsing dot while the job runs. The Overview's Agent section says "The agent is working · N pairs still to look at" while it runs, and "N pairs waiting for the agent" when the switch is on and there is backlog.
• ReviewCounts.agent_running / agent_queue and ReviewSummary.agent.running / queue carry the state; the page refreshes on the review events the job already emits after each cluster.

Verified

• Store test: with a running govern job row, a locked pair refuses decide_review; releasing the locks or removing the job unlocks it; queue_len counts the pairs the agent has not looked at.
• On the dev server against the bench base during a run: the Overview line reads "The agent is working · 449 pairs still to look at", the rail dot pulses, and a POST to decide a locked pair returns 409 with the message.

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