Inject user follow-up messages into the running agent loop at turn
boundaries instead of queueing them for a new run. This preserves
context so the LLM sees both tool results and user follow-ups together.
- Add InjectedMessage type and drainInjectChannel helper
- Add InjectCh to ActiveRun with buffered channel (cap=5)
- Drain injection channel at two points in agent loop (after tool
results and before no-tool-calls exit)
- Route steer/new_task intents to InjectMessage with scheduler fallback
- WebSocket: inject into running loop when session is busy
- Remove IntentClassify config toggle (always on)
- Web UI: show send + stop buttons side by side during agent run
- i18n: add injection acknowledgment messages (en/vi/zh)
