fix(workflow): use GitHub Releases + Docker Hub hybrid for version detection

Root cause: Docker Hub sorts tags by last_updated (non-monotonic).
Some versions (e.g. 2.24.x) had older last_updated than 2.23.4, so they
fell off page 1 and were never detected. Workflow stuck at 2.22.2.

Fix:
- Primary source: GitHub Releases API (stable releases, per_page=30)
- Secondary source: Docker Hub page 1 (catches releases before GitHub promotes them)
- Take the maximum of both candidates
- Sanity check: LATEST must be >= CURRENT to prevent false downgrades
- Added --retry 3 and .draft==false filter to GitHub source

Also bump chart and image to 2.26.3 (manual catch-up from 2.22.2).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

README.md

@@ -237,8 +237,10 @@ helm upgrade my-n8n cafepieters/n8n \
 
 | バージョン | n8n | 変更内容 |
 |---|---|---|
-| バージョン | n8n | 変更内容 |
-|---|---|---|
+| 2.26.3 | 2.26.3 | 自動更新ワークフロー修正（GitHub Releases + Docker Hub ハイブリッドソース） |
+| 2.22.2 | 2.22.2 | 自動更新 |
+| 2.21.3 | 2.21.3 | 自動更新 |
+| 2.20.6 | 2.20.6 | 自動更新 |
 | 2.19.2-c | 2.19.2 | 暗号化キー復旧手順を README に追加 |
 | 2.19.2-b | 2.19.2 | 暗号化キーを `lookup` で既存 Secret から維持、`helm upgrade` での再生成を防止 |
 | 2.19.2-a | 2.19.2 | `N8N_SECURE_COOKIE` を Ingress/TLS 設定から自動判定（HTTP/LoadBalancer 環境対応） |