fix(core): follow merged tombstones in the position lookup

A merged child still holds its (parent_id, position) slot under the unique
index, so skipping tombstones made every retry collide. Follow the tombstone
to the surviving work instead.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/core/src/matching.ts

@@ -4,6 +4,7 @@ import type { Db } from "@sttracker/db/db";
 import { newId, nowIso } from "@sttracker/db/ids";
 import {
   createWorkIn,
+  finalWorkId,
   findWorkByExternalIdsIn,
   normalizePosition,
   resolveAliasIn,
@@ -85,12 +86,13 @@ async function resolve(k: K, userId: string | null, hint: WorkHint, actor: strin
       .select("id")
       .where("parent_id", "=", parentId)
       .where("position", "=", position)
-      .where("merged_into_id", "is", null)
       .where("owner_user_id", "is", null)
       .executeTakeFirst();
     if (child) {
-      await updateWorkFromHintIn(k, child.id, hint, actor);
-      return { workId: child.id, created: false, draft: false };
+      // A merged tombstone still holds its (parent_id, position) slot: follow it to the surviving work.
+      const workId = await finalWorkId(k, child.id);
+      await updateWorkFromHintIn(k, workId, hint, actor);
+      return { workId, created: false, draft: false };
     }
     return { workId: await createWorkIn(k, { hint, parentId }, actor), created: true, draft: false };
   }