launcher: reap stale front/engine trees before start

Engines run in their own sessions, so supervisord's killasgroup cannot
reach them; teardown relies on the front's shutdown handler + the
engines' PDEATHSIG. That chain leaked a full front+engine tree across a
restart — two trees then fought over one GPU (phantom VRAM, evictions
freeing nothing, duplicated model workers, requests served by one tree
while logs came from the other). The launcher now pkills any surviving
coderai-front/engine/dinov2-embed processes before starting: at that
point none can legitimately exist.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014S8VtAvG499SsCbeESRK7V