Coordinator settle tool, agent turn rule, continuation result delivery

[nickrroberts]

Summary

• cp_agent_settle (coordinator only, agent, optional archive): settles an idle agent with the same thread.settle command the UI sends. Busy agent, standing agent, or non-coordinator caller gets a clear error.
• Coordinator prompt: settle agents once their work is complete and merged, or when one-off and reported.
• Agent prompt: never end a turn just to wait for a command, test run, or sub-agent. Standing agents keep the cp_agent_create exception, since their agents report as messages.
• Agent browser rule: load the preview_* tools and call preview_status/preview_open before concluding the built-in browser is unavailable.
• Lost final report fix (AgentCompletionReactor): a request whose own turn ended with no reply and was answered by a later turn no message started (SDK resume or wake-up) now delivers that reply once, as cp-push:<agent>:cp-turn:<turnId>. Before, the coordinator got "finished (no final message)" and the real report was never pushed, because that turn has no request row (pending_message_id NULL).
• Work log shows "Settled " for the new tool.

Test plan

• handlers tests: settle idle success, archive, busy, engine-blocked, not-yours, coordinator-only, standing
• assistantRuntime.test.ts: settle rule, turn rule (and standing exception), preview rule
• reactor tests: empty request then continuation reply delivered once; no double delivery when the reply is already in the request result; no push for a later wake-up after a request that reported
• Full server suite: 51 failed in 10 files, identical to main (ThreadSettlementReactor 37 confirmed failing on clean main); typecheck clean for server and client-runtime; lint warnings only

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