ci: remove mirror-to-knot workflow (superseded by lith systemd timer)
Tangled's knot runs .github/workflows/*.yml as pipelines too, so
this file was firing failure notifications there. GitHub Actions
is billing-locked on the repo so it wasn't running there anyway.
Lith's /opt/ac-mirror/mirror.sh + ac-mirror.timer (every 60s,
bidirectional) handles the actual mirroring now. That makes this
workflow dead weight causing email noise.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>