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>

.env.example

@@ -195,3 +195,41 @@ MCP_DOCMOST_PASSWORD=
 # FAILS CLOSED if Redis is unavailable (default: 1,000,000 tokens per workspace
 # per rolling day).
 # SHARE_AI_WORKSPACE_TOKEN_BUDGET_PER_DAY=1000000
+
+# --- GIT-SYNC (native two-way Docmost <-> git Markdown sync) ---
+# Master switch. Off by default. When 'true', GIT_SYNC_SERVICE_USER_ID below is
+# REQUIRED (the service account that git-originated create/move/rename/delete are
+# attributed to) — the server refuses to boot with sync enabled and no user id.
+# GIT_SYNC_ENABLED=false
+#
+# Serve the per-space vaults over smart-HTTP (the /git host). Defaults to
+# GIT_SYNC_ENABLED when unset.
+# GIT_SYNC_HTTP_ENABLED=false
+#
+# REQUIRED when GIT_SYNC_ENABLED=true: id of the user that git-originated page
+# operations (create / move / rename / delete) are attributed to.
+# GIT_SYNC_SERVICE_USER_ID=
+#
+# Where the per-space bare repos / working vaults live.
+# Defaults to "<DATA_DIR or ./data>/git-sync".
+# GIT_SYNC_DATA_DIR=
+#
+# Optional remote URL template to mirror each space's vault to (e.g. a git host).
+# Leave unset to keep vaults local-only.
+# GIT_SYNC_REMOTE_TEMPLATE=
+#
+# Path to the SSH private key used when pushing to GIT_SYNC_REMOTE_TEMPLATE.
+# GIT_SYNC_SSH_KEY_PATH=
+#
+# Poll-safety interval in ms — the cadence of the background reconcile cycle
+# (default: 15000).
+# GIT_SYNC_POLL_INTERVAL_MS=15000
+#
+# Debounce window in ms for collapsing bursts of page edits into one sync cycle
+# (default: 2000).
+# GIT_SYNC_DEBOUNCE_MS=2000
+#
+# Defense-in-depth absolute cap on soft-deletes applied per push cycle
+# (default: 5). A non-convergent / phantom-absence cycle can never trash more
+# than this many pages without an explicit override.
+# GIT_SYNC_MAX_DELETES_PER_CYCLE=5