v1.6.8

Fix the v1.6.7 self-update death loop.

v1.6.7's node binary self-reported the source-hardcoded "1.6.6" (release.yml
never injected the tag via -ldflags), so every node that installed v1.6.7
saw a version mismatch on its next refresh and re-triggered install.sh,
looping ~1 install/second until the master's desiredAgentVersion was cleared.

Fixes:
- release.yml now bakes the tag into main.Version at link time.
- updater cooldown is persisted to /opt/wolf/.last-update-attempt so a
  future version-drift bug can burn install.sh once every 5min at worst.
- Source default bumped to 1.6.8.

Rolling out to v1.6.8 will end the loop for any node still stuck on 1.6.7.