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>

docs/TODO.md

@@ -23,6 +23,7 @@ Small, specific, independently actionable. Remove only when actually done.
     file, and compare steps and token use.
 
 ## Known issues
+- [ ] Admin merge does not merge colliding children recursively; colliding children stay under the tombstoned parent.
 - [ ] Matching: a shared parent resolved without external IDs or position (title/year reuse in
   `packages/core/src/matching.ts`) has no unique key, so concurrent matches can create it twice.
 - [ ] `bun run typecheck` fails with TS18003 until source files exist under `apps/`, `packages/` or `plugins/`.