On first agent deployment, the container boots before hive-c0re has
provisioned the forge-token. forge-avatar-sync was exiting early with
"no forge-token found", and RemainAfterExit=true prevented systemd
from ever re-running it — avatar never got uploaded until the next
container reboot.
Add a systemd.paths.forge-avatar-sync unit (PathExistsGlob on the
forge-token file) to re-fire the service once the token arrives, and
set RemainAfterExit=false to allow the re-fire. Mirrors the existing
matrix-avatar-sync pattern exactly.
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