Auto-reconcile subrepo history when josh filter changes

When the exclude list changes, josh-proxy recomputes filtered history
with new SHAs, breaking common ancestry with the subrepo. Instead of
requiring a manual reset (force-push), forward sync now detects the
filter change and creates a reconciliation merge commit that connects
the old and new histories — no force-push, no re-clone needed.

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

bin/josh-sync

@@ -208,10 +208,20 @@ _sync_direction() {
         fi
       fi
 
-      # Run sync
+      # Check for filter change (forward only — reverse uses same filter)
       local result
       if [ "$direction" = "forward" ]; then
-        result=$(forward_sync)
+        local prev_filter
+        prev_filter=$(echo "$state" | jq -r '.last_forward.josh_filter // empty')
+
+        if [ -n "$prev_filter" ] && [ "$prev_filter" != "$JOSH_FILTER" ]; then
+          log "WARN" "Josh filter changed — reconciling histories"
+          log "INFO" "Old: ${prev_filter}"
+          log "INFO" "New: ${JOSH_FILTER}"
+          result=$(reconcile_filter_change)
+        else
+          result=$(forward_sync)
+        fi
       else
         result=$(reverse_sync)
       fi
@@ -240,8 +250,9 @@ _sync_direction() {
           --arg s_sha "${subrepo_sha_now:-}" \
           --arg ts "$(date -u +%Y-%m-%dT%H:%M:%SZ)" \
           --arg status "$result" \
+          --arg filter "$JOSH_FILTER" \
           --argjson prev "$state" \
-          '$prev + {last_forward: {mono_sha:$m_sha, subrepo_sha:$s_sha, timestamp:$ts, status:$status}}')
+          '$prev + {last_forward: {mono_sha:$m_sha, subrepo_sha:$s_sha, timestamp:$ts, status:$status, josh_filter:$filter}}')
       else
         local mono_sha_now
         mono_sha_now=$(git rev-parse "origin/${branch}" 2>/dev/null || echo "")