fix(kanban): treat dashboard event-stream cancellation as normal shutdown

[Fearvox]

Summary

Stopping hermes dashboard with Ctrl-C while the Kanban dashboard is open can print an ASGI traceback from the dashboard event stream. The stream handler is cancelled during normal shutdown while sleeping in its poll loop, but asyncio.CancelledError is surfaced as an application error.

Repro

1. Run hermes --yolo dashboard.
2. Open the dashboard Web UI.
3. Stop the dashboard with Ctrl-C while the page is still connected.

Observed

Uvicorn prints an application traceback ending in the Kanban dashboard event stream:

plugins/kanban/dashboard/plugin_api.py, in stream_events
    await asyncio.sleep(_EVENT_POLL_SECONDS)
asyncio.exceptions.CancelledError

There can also be a lifespan cancellation traceback after Ctrl-C.

Expected

Dashboard shutdown or browser disconnect should be quiet, or at most debug-level noise. This is a normal cancellation path, not an application failure.

Suggested fix

Handle asyncio.CancelledError beside WebSocketDisconnect and return without warning:

except WebSocketDisconnect:
    return
except asyncio.CancelledError:
    return

Notes

This does not look like data loss or task corruption. It is control-plane polish: normal dashboard exit currently looks like a runtime failure, which makes operator debugging noisier than it needs to be.