fix(#370): add savePageVersion to DocmostClientMethod union (server in-app tool parity)

The Stage-A save_page_version tool's client method must be listed in the
DocmostClientMethod union so Pick<DocmostClient, DocmostClientMethod> exposes it
to the in-app tool adapter; the shared-tool-specs contract spec asserts parity.
Mirrors the existing restorePageVersion entry.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/server/src/core/ai-chat/tools/docmost-client.loader.ts

@@ -58,6 +58,7 @@ type DocmostClientMethod =
   | 'sharePage'
   | 'unsharePage'
   | 'restorePageVersion'
+  | 'savePageVersion'
   | 'transformPage'
   | 'stashPage'
   // --- write (image / footnote), in-app since #410 ---