feat(sync): add deterministic maintenance-thread dedup key

packages/backend/convex/maintenanceDedup.ts

@@ -0,0 +1,21 @@
+/**
+ * Pure, deterministic dedup key for maintenance threads.
+ *
+ * Derives a stable key from the merge-base + head SHAs so that repeated
+ * scheduled sync runs against the same upstream state collapse onto a single
+ * maintenance thread instead of creating a new one each run. NEVER involves
+ * Date.now()/random — the key must be identical for identical inputs.
+ *
+ * Returns `null` when the head SHA is missing/empty (unresolvable head → the
+ * caller cannot dedup and should SKIP thread creation rather than mint a
+ * unique key).
+ */
+export const maintenanceDedupKey = (input: {
+  mergeBaseSha?: string;
+  headSha?: string;
+}): string | null => {
+  const head = input.headSha?.trim();
+  if (!head) return null;
+  const base = input.mergeBaseSha?.trim() || 'nobase';
+  return `${base}:${head}`;
+};