Refactor control-plane session runs async

[roackb2]

Summary

• add sessionSendPromptAsync for interactive clients so submit acknowledgement is separate from long-running agent work
• persist accepted user prompts through core session ownership before streaming activity starts
• update web-v2 and cli-v2 to consume async runs, keep typing available during active runs, and rely on shared API state instead of local optimistic user messages

Validation

• yarn -s vitest run src/tests/integration/control-plane/session-lifecycle.test.ts src/tests/unit/core/chat-turn-runtime.test.ts src/tests/unit/core/chat-turn-persistence.test.ts src/tests/unit/control-plane/session-cancel.test.ts src/tests/unit/cli-v2/control-plane-session-store.test.ts src/tests/unit/control-plane/session-message-service.test.ts
• yarn -s typecheck
• yarn -s build