ci: GitHub → knot mirror workflow
Fires on every push to main and re-pushes HEAD to
knot.aesthetic.computer:aesthetic.computer/core, so any commit
that lands on GitHub (PR merges, bot pushes, etc.) shows up on
knot within ~30 seconds.
Knot is production for lith; without this, a PR merged on
GitHub would never reach lith until someone local compushed.
Requires repo secret TANGLED_KNOT_SSH_KEY (set via vault's
home/.ssh/tangled private key, authorized on the knot repo).
Reverse direction (knot → GitHub) still relies on the dual-
pushurl convention in the local git config — Tangled knot
doesn't expose post-receive hooks for us to install on.
Also replaces the .github symlink (pointing to a local-only
path /home/me/aesthetic-computer/modes that broke GitHub
Actions) with a real directory.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>