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}`;
+};

packages/backend/convex/schema.ts

@@ -797,6 +797,7 @@ const applicationTables = {
     ),
     ignoredCommitShas: v.optional(v.array(v.string())),
     ignoredReason: v.optional(v.string()),
+    dedupKey: v.optional(v.string()),
     createdAt: v.number(),
     updatedAt: v.number(),
     resolvedAt: v.optional(v.number()),
@@ -805,7 +806,8 @@ const applicationTables = {
     .index('by_owner_status', ['ownerId', 'status'])
     .index('by_spoon', ['spoonId'])
     .index('by_source', ['ownerId', 'source'])
-    .index('by_created', ['createdAt']),
+    .index('by_created', ['createdAt'])
+    .index('by_spoon_dedup', ['spoonId', 'dedupKey']),
   threadMessages: defineTable({
     threadId: v.id('threads'),
     ownerId: v.id('users'),