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>

lib/sync.sh

@@ -128,6 +128,85 @@ ${BOT_TRAILER}: forward/${mono_branch}/$(date -u +%Y-%m-%dT%H:%M:%SZ)" >&2
   fi
 }
 
+# ─── Filter Change Reconciliation ─────────────────────────────────
+# When the josh filter changes (e.g., exclude patterns added/removed),
+# josh-proxy recomputes filtered history with new SHAs. This creates a
+# merge commit on the subrepo that connects old and new histories,
+# re-establishing shared ancestry without a destructive force-push.
+# Returns: reconciled | lease-rejected
+
+reconcile_filter_change() {
+  local mono_branch="$SYNC_BRANCH_MONO"
+  local subrepo_branch="$SYNC_BRANCH_SUBREPO"
+  local work_dir
+  work_dir=$(mktemp -d)
+  # shellcheck disable=SC2064  # Intentional early expansion — work_dir is local
+  trap "rm -rf '$work_dir'" EXIT
+
+  log "INFO" "=== Filter change reconciliation: ${mono_branch} ==="
+
+  # 1. Clone subrepo
+  git clone "$(subrepo_auth_url)" \
+    --branch "$subrepo_branch" --single-branch \
+    "${work_dir}/subrepo" || die "Failed to clone subrepo"
+
+  cd "${work_dir}/subrepo" || exit
+  git config user.name "$BOT_NAME"
+  git config user.email "$BOT_EMAIL"
+
+  local subrepo_head
+  subrepo_head=$(git rev-parse HEAD)
+  log "INFO" "Subrepo HEAD: ${subrepo_head:0:12}"
+
+  # 2. Fetch josh-proxy filtered view (new filter)
+  git remote add josh-filtered "$(josh_auth_url)"
+  git fetch josh-filtered "$mono_branch" || die "Failed to fetch from josh-proxy"
+
+  local josh_head josh_tree
+  josh_head=$(git rev-parse "josh-filtered/${mono_branch}")
+  # shellcheck disable=SC1083  # {tree} is git syntax, not shell brace expansion
+  josh_tree=$(git rev-parse "josh-filtered/${mono_branch}^{tree}")
+  log "INFO" "Josh-proxy HEAD (new filter): ${josh_head:0:12}"
+
+  # 3. Check if trees are already identical (filter change had no effect)
+  local subrepo_tree
+  # shellcheck disable=SC1083
+  subrepo_tree=$(git rev-parse "HEAD^{tree}")
+  if [ "$josh_tree" = "$subrepo_tree" ]; then
+    log "INFO" "Trees identical after filter change — no reconciliation needed"
+    echo "skip"
+    return
+  fi
+
+  # 4. Create merge commit: subrepo HEAD + josh-proxy HEAD, with josh-proxy's tree
+  local merge_commit
+  merge_commit=$(git commit-tree "$josh_tree" \
+    -p "$subrepo_head" \
+    -p "$josh_head" \
+    -m "Sync: filter configuration updated
+
+${BOT_TRAILER}: filter-change/${mono_branch}/$(date -u +%Y-%m-%dT%H:%M:%SZ)")
+
+  git reset --hard "$merge_commit" >&2
+  log "INFO" "Created reconciliation merge: ${merge_commit:0:12}"
+
+  # 5. Record lease and push
+  local subrepo_sha
+  subrepo_sha=$(subrepo_ls_remote "$subrepo_branch")
+
+  if git push \
+    --force-with-lease="refs/heads/${subrepo_branch}:${subrepo_sha}" \
+    "$(subrepo_auth_url)" \
+    "HEAD:refs/heads/${subrepo_branch}"; then
+
+    log "INFO" "Filter change reconciled — shared ancestry re-established"
+    echo "reconciled"
+  else
+    log "WARN" "Force-with-lease rejected — subrepo changed during reconciliation"
+    echo "lease-rejected"
+  fi
+}
+
 # ─── Reverse Sync: Subrepo → Monorepo ──────────────────────────────
 #
 # Always creates a PR on the monorepo — never pushes directly.