docs: ADR 0001's spike is kept by its developer, not published

The ADR named the branch spike/offline-sync as its evidence as though
anyone could look at it, but it was never pushed; with the repository
public, it says so, and that what the spike established is written down
in the ADR itself.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

docs/adr/0001-offline-sync.md

@@ -2,8 +2,9 @@
 
 - **Status:** accepted
 - **Date:** 2026-09-27
-- **Evidence:** branch `spike/offline-sync` (never merged), tests in
-  `halkyon_learn_server/test/spike/`, run against a live server on Serverpod 4.0.2.
+- **Evidence:** a spike on a branch of its own, `spike/offline-sync`, never merged and not
+  published (its developer keeps it): tests in `halkyon_learn_server/test/spike/`, run
+  against a live server on Serverpod 4.0.2. What they established is below.
 - **Amended by:** [ADR 0002](0002-no-end-to-end-encryption.md) (2026-09-29): there is no
   end-to-end encryption, so the premise below that the server syncs data it cannot read no
   longer holds. The decision and its rules stand. Deleted content is redacted before