docs(git-sync): document GIT_SYNC_* env vars; fix stale/non-English comments (PR #119 review)

Addresses the documentation/convention warnings from the #119 review:
- .env.example: add the GIT-SYNC block (9 GIT_SYNC_* vars with defaults), noting
  GIT_SYNC_SERVICE_USER_ID is required when sync is enabled.
- yjs-body-merge.ts: translate the Russian review note in the docstring to
  English (comments-only-in-English rule).
- persistence.extension.ts: correct the stale "git-sync writes are full-body
  replaces" rationale — a git-sync write is now a block-level merge into the live
  doc, which is why it is debounced like a human edit rather than snapshotted.
- history-item.tsx: the GitSyncBadge version is created on the PUSH path (writing
  the git body back into the doc), not by the pull — fix the comment.
- edit-space-form.tsx: log the raw error in the git-sync toggle catch instead of
  swallowing it (AGENTS.md).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

apps/client/src/features/space/components/edit-space-form.tsx

@@ -52,6 +52,9 @@ export function EditSpaceForm({ space, readOnly }: EditSpaceFormProps) {
       });
     } catch (err) {
       setGitSyncEnabled(previous); // revert on failure
+      // The mutation surfaces a toast via onError; still log the raw error so it
+      // is not silently swallowed (AGENTS.md).
+      console.error("Failed to toggle git-sync for space", err);
     }
   };