Fix reverse sync missing human commits when mono-filtered advances

--ancestry-path returns empty when mono-filtered/main has advanced
past the last forward sync (any monorepo commit causes josh to create
a new filtered SHA not in the subrepo's ancestry). Use git merge-base
to find the last connected point instead.

Also return skip-dirty (not skip) when trees differ but no human
commits found, preventing state from being updated on false skips
which would permanently lose unsynced commits.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

bin/josh-sync

@@ -259,6 +259,10 @@ _sync_direction() {
         echo "::error::Target ${target_name}, branch ${branch}: josh rejected push — check proxy logs"
         continue
       fi
+      if [ "$result" = "skip-dirty" ]; then
+        log "WARN" "Trees differ but no human commits — skipping state update (will retry)"
+        continue
+      fi
 
       # Update state (only on success)
       local new_state