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>

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