fix(git-sync): screen non-page files out of PUSH (CRITICAL — review)

Self-review of phase 3 caught a data-corruption regression: nativeMeta always
supplies the run's spaceId, so the planner's 'create-without-spaceId' skip — which
had doubled as the only filter for non-page files — went dead. An ADDED
.obsidian/*.json, attachment, or dotfile (committed to the vault, no .gitignore)
would then be classified as a CREATE: a junk Docmost page, plus a gitmost_id
frontmatter written INTO the file, corrupting it.

Fix: isPageFile(path) — a .md file with NO dot-segment anywhere — and filter the
diff to page files at the very top of computePushActions, BEFORE any
classification, so non-page A/M/D/R are ignored (design §Адопция). 2 unit tests
pin it (.obsidian/json, attachment, dotfile, dot-segment, .md dotfile all ignored;
real pages still created). 614 engine tests green.

Also: refreshed stale docmost:meta comments to gitmost_id (review SUGGESTION), and
documented the deferred adoption frontmatter-preservation gap (review WARNING) in
page-file.ts + the design doc (do NOT roll native onto a real vault with Obsidian
properties until phase 4 round-trips them).

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

docs/backlog/git-sync-thin-meta.md

@@ -122,6 +122,12 @@ Obsidian резолвит `[[Заметка]]` по **basename** (не по по
    пути (`parentFolderFile` folder-note-aware). CREATE пишет `gitmost_id` обратно;
    UPDATE шлёт чистое тело (без frontmatter) на обе стороны 3-way merge. (готово)
 4. Адопция голых файлов/папок (частично в фазе 3: файл без `gitmost_id` → create).
+   ВАЖНО: тут же сохранить пользовательский frontmatter (Obsidian properties) при
+   адопции — `parsePageFile` сейчас срезает ведущий frontmatter даже без
+   `gitmost_id`, а write-back пишет только `gitmost_id`; нужно врезать `gitmost_id`
+   в существующий frontmatter и сохранять остальные поля И при write-back, И при
+   следующем pull (иначе pull перезатрёт). До этого native-формат НЕ катить на
+   реальный Obsidian-волт с properties.
 5. Чистка: выпилить старый `docmost:meta` формат-код целиком.
 6. Ссылки: конвертер Docmost-mention ↔ `[[wikilink]]` + переписывание при retitle.