Fix filter reconciliation for pre-v1.2 state and unrelated histories

Three bugs found during first CI run after enabling :exclude:

- Derive old filter (:/subfolder) when state has no josh_filter stored
  (pre-v1.2 upgrade path)
- Detect unrelated histories in forward_sync() and fall back to
  reconcile_filter_change() instead of creating a useless conflict PR
- Skip state update on conflict result (prevents storing wrong filter
  and mono SHA that blocks retries)

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

bin/josh-sync

@@ -214,6 +214,18 @@ _sync_direction() {
         local prev_filter
         prev_filter=$(echo "$state" | jq -r '.last_forward.josh_filter // empty')
 
+        # If no filter stored (pre-v1.2 state) but a previous sync exists,
+        # the old filter was the simple :/subfolder (before exclude was added)
+        if [ -z "$prev_filter" ]; then
+          local prev_mono_sha
+          prev_mono_sha=$(echo "$state" | jq -r '.last_forward.mono_sha // empty')
+          if [ -n "$prev_mono_sha" ]; then
+            local subfolder
+            subfolder=$(echo "$TARGET_JSON" | jq -r '.subfolder')
+            prev_filter=":/${subfolder}"
+          fi
+        fi
+
         if [ -n "$prev_filter" ] && [ "$prev_filter" != "$JOSH_FILTER" ]; then
           log "WARN" "Josh filter changed — reconciling histories"
           log "INFO" "Old: ${prev_filter}"
@@ -225,6 +237,13 @@ _sync_direction() {
       else
         result=$(reverse_sync)
       fi
+      # If forward sync hit unrelated histories, fall back to reconciliation
+      if [ "$result" = "unrelated" ]; then
+        log "WARN" "Unrelated histories detected — falling back to filter reconciliation"
+        result=$(reconcile_filter_change)
+        log "INFO" "Reconciliation result: ${result}"
+      fi
+
       log "INFO" "Result: ${result}"
 
       # Handle warnings
@@ -234,6 +253,7 @@ _sync_direction() {
       fi
       if [ "$result" = "conflict" ]; then
         echo "::warning::Target ${target_name}, branch ${branch}: merge conflict — PR created on subrepo"
+        continue
       fi
       if [ "$result" = "josh-rejected" ]; then
         echo "::error::Target ${target_name}, branch ${branch}: josh rejected push — check proxy logs"

lib/sync.sh

@@ -11,7 +11,7 @@
 
 # ─── Forward Sync: Monorepo → Subrepo ──────────────────────────────
 #
-# Returns: fresh | skip | clean | lease-rejected | conflict
+# Returns: fresh | skip | clean | lease-rejected | conflict | unrelated
 
 forward_sync() {
   local mono_branch="$SYNC_BRANCH_MONO"
@@ -97,7 +97,14 @@ ${BOT_TRAILER}: forward/${mono_branch}/$(date -u +%Y-%m-%dT%H:%M:%SZ)" >&2
     fi
 
   else
-    # Conflict!
+    # Check: unrelated histories (filter change) vs normal merge conflict
+    if ! git merge-base "subrepo/${subrepo_branch}" "$mono_head" >/dev/null 2>&1; then
+      log "INFO" "No common ancestor — histories are unrelated (filter change?)"
+      echo "unrelated"
+      return
+    fi
+
+    # Normal merge conflict
     local conflicted
     conflicted=$(git diff --name-only --diff-filter=U 2>/dev/null || echo "(unknown)")
     git merge --abort