fix(coding-agent): acknowledge prompts queued during streaming

[nnunley]

Summary

• acknowledge a prompt immediately after it is admitted while the agent is streaming
• distinguish steering queued for the next model boundary from follow-up queued until the run finishes
• avoid describing an idle prompt as queued

The acknowledgement is emitted by the interactive client after admission, so it does not depend on the active model producing another token. #1181 tracks the broader received/applied/discarded lifecycle across all clients.

Testing

• npx tsx ../../node_modules/vitest/dist/cli.js --run test/interactive-mode-status.test.ts (154 tests passed)
• npm run check

Note

Acknowledge prompts queued during streaming in interactive mode

When a prompt is submitted while the agent is streaming, InteractiveMode.setupEditorSubmitHandler now immediately shows a status message confirming the submission was received and queued. The message varies by streamingBehavior: 'steer' reports queuing until the next model boundary; 'followUp' reports queuing until the current run finishes.

^{Macroscope summarized 1a298f9.}