[P3] Reaper notifies channels before killSession — race condition on shutdown

## Problem

Reaper calls channels.sessionEnded() before killSession(). This means channels are notified that a session has ended before it is actually killed.

## Verified Status

NEW — NOT TRACKED

## Impact

Race condition — channel handlers may try to interact with a session that is not yet fully shut down.

## Environment

- Aegis version: latest (main)
- Install method: npm/global