Fix reconciliation parent order and add reverse sync tree check

- Swap parent order in reconcile_filter_change(): josh-filtered must
  be first parent so josh can follow first-parent traversal to map
  history back to the monorepo. Old subrepo history on parent 2.
- Add tree comparison in reverse_sync() before commit detection:
  if subrepo tree matches josh-filtered tree, skip immediately.
  Prevents false positive PRs after reconciliation.

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

lib/sync.sh

@@ -185,11 +185,13 @@ reconcile_filter_change() {
     return
   fi
 
-  # 4. Create merge commit: subrepo HEAD + josh-proxy HEAD, with josh-proxy's tree
+  # 4. Create merge commit: josh-proxy HEAD (first parent) + subrepo HEAD, with josh-proxy's tree
+  # Josh follows first-parent traversal — josh-filtered MUST be first so josh can map
+  # the history back to the monorepo. Old subrepo history hangs off parent 2.
   local merge_commit
   merge_commit=$(git commit-tree "$josh_tree" \
-    -p "$subrepo_head" \
     -p "$josh_head" \
+    -p "$subrepo_head" \
     -m "Sync: filter configuration updated
 
 ${BOT_TRAILER}: filter-change/${mono_branch}/$(date -u +%Y-%m-%dT%H:%M:%SZ)")
@@ -242,7 +244,20 @@ reverse_sync() {
   git remote add mono-filtered "$(josh_auth_url)"
   git fetch mono-filtered "$mono_branch" || die "Failed to fetch from josh-proxy"
 
-  # 3. Find new human commits (excludes bot commits from forward sync)
+  # 3. Compare trees — skip if subrepo matches josh-filtered view
+  local subrepo_tree josh_tree
+  # shellcheck disable=SC1083  # {tree} is git syntax, not shell brace expansion
+  subrepo_tree=$(git rev-parse "HEAD^{tree}")
+  # shellcheck disable=SC1083
+  josh_tree=$(git rev-parse "mono-filtered/${mono_branch}^{tree}")
+
+  if [ "$subrepo_tree" = "$josh_tree" ]; then
+    log "INFO" "Subrepo tree matches josh-filtered view — nothing to sync"
+    echo "skip"
+    return
+  fi
+
+  # 4. Find new human commits (excludes bot commits from forward sync)
   local human_commits
   human_commits=$(git log --ancestry-path "mono-filtered/${mono_branch}..HEAD" \
     --oneline --invert-grep --grep="^${BOT_TRAILER}:" 2>/dev/null || echo "")