fix(acp): wait for settled prompt after Stop; skip SESSION_BUSY auto-retry

[metalmon]

Summary

• After Stop, await the in-flight session/prompt to settle before starting another prompt on the same ACP session, so a quick follow-up send does not race the agent's still-busy turn slot.
• Treat ACP "active prompt turn" / SESSION_BUSY errors as non-retryable in chat auto-retry (same idea as rate limits), so blind retries do not amplify the race.

Fixes #1094

Context

Stop already sends session/cancel and tears down the local SSE stream. Some ACP agents (e.g. Zeroclaw) only clear the busy slot when the cancelled turn fully exits. The UI becomes free immediately, so the next message can hit Session already has an active prompt turn and auto-retry makes it worse.

Test plan

• bun test src/acp/acp-adapter.test.ts src/lib/error-utils.test.ts
• Stop mid-turn on a remote ACP agent, then send again once the prior prompt has settled — no SESSION_BUSY / active-prompt-turn error from racing the next session/prompt
• If SESSION_BUSY still surfaces (slow agent cleanup), confirm chat does not auto-retry in a loop