Fix reverse sync false positive after filter reconciliation

Add --ancestry-path to git log in reverse_sync() to prevent old
subrepo history from leaking through reconciliation merge parents.
Without this, every old subrepo commit appears as a "human commit"
triggering a spurious 0-commit PR on the monorepo.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/sync.sh

@@ -244,7 +244,7 @@ reverse_sync() {
 
   # 3. Find new human commits (excludes bot commits from forward sync)
   local human_commits
-  human_commits=$(git log "mono-filtered/${mono_branch}..HEAD" \
+  human_commits=$(git log --ancestry-path "mono-filtered/${mono_branch}..HEAD" \
     --oneline --invert-grep --grep="^${BOT_TRAILER}:" 2>/dev/null || echo "")
 
   if [ -z "$human_commits" ]; then