2configs: migrate syncthing and zerotier from facts to vars
- Update syncthing peer discovery to read from vars/per-machine//syncthing/syncthing.pub/value
- Update zerotier host discovery to read from vars/per-machine//zerotier/zerotier-ip/value
- Update syncthing key/cert references to use config.clan.core.vars.generators.syncthing.files paths