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/server/src/integrations/git-sync/services/yjs-body-merge.ts

@@ -5,8 +5,8 @@ import { diff3Plan } from './three-way-merge';
 /**
  * Block-level merge of an incoming (git) page body into a LIVE Yjs document,
  * replacing the previous full-body "delete everything + re-insert" write that
- * clobbered concurrent human edits on every sync (review #5 — "запись делать
- * через мерж").
+ * clobbered concurrent human edits on every sync (review #5 — "do the write as a
+ * merge").
  *
  * Strategy: diff the two documents at TOP-LEVEL BLOCK granularity (an LCS over a
  * canonical structural serialization of each block) and apply only the minimal