feat(git-sync): phase 2b — PULL writes native gitmost_id frontmatter

PULL now serializes each page as the native-Obsidian format (serializePageFile:
a minimal gitmost_id frontmatter + the fixpoint markdown body) instead of the
heavy docmost:meta envelope. title/parent/space are derived (filename / folder /
repo), so only the pageId is persisted. readExisting recovers identity from the
gitmost_id frontmatter (parsePageFile) instead of docmost:meta.

Extracted stabilizePageBody() (the export->import->export fixpoint, no meta) so
the native writer and the legacy serializer share the same deterministic body —
re-pulls of an unchanged page stay byte-identical (loop-guard).

Tests: read-existing fixtures rewritten to gitmost_id; apply-pull asserts the
written text is native frontmatter and carries NO docmost:meta (regression
guard). 611 engine tests green.

NOTE: PUSH still reads docmost:meta — the end-to-end cycle is intentionally NOT
runnable until phase 3 (PUSH reads frontmatter + derives title/parent from path)
lands; no vault is wiped/deployed until then.

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

packages/git-sync/src/engine/pull.ts

@@ -8,7 +8,7 @@
  *   2. checkout docmost
  *   3. fetch the live tree (listSpaceTree -> {pages, complete}) -> compute the
  *      desired `live` files (relPath via the pure sanitize/disambiguation layout)
- *   4. parse `existing` tracked .md files (pageId + relPath from docmost:meta)
+ *   4. parse `existing` tracked .md files (pageId + relPath from gitmost_id frontmatter)
  *   5. plan = planReconciliation(live, existing)   (pure, SPEC §5/§8); toDelete
  *      is absence-only, moves are separate
  *   6. decideAbsenceDeletions: SUPPRESS absence deletions on an incomplete tree
@@ -32,7 +32,7 @@
  */
 import { dirname } from "node:path";
 import { sep } from "node:path";
-import { parseDocmostMarkdown } from "../lib/index";
+import { parsePageFile, serializePageFile } from "../lib/page-file";
 import type { GitSyncClient } from "./client.types";
 import { buildVaultLayout, type PageNode } from "./layout";
 import {
@@ -48,7 +48,7 @@ import {
   type MovedEntry,
   type DeletionDecision,
 } from "./reconcile";
-import { stabilizePageFile, type PageMeta } from "./stabilize";
+import { stabilizePageBody } from "./stabilize";
 
 // Engine-only mirror branch (SPEC §5): the engine writes here, humans never do.
 const DOCMOST_BRANCH = "docmost";
@@ -85,15 +85,15 @@ export interface ReadExistingDeps {
 }
 
 /**
- * Read every tracked .md file in the vault and parse its `docmost:meta` to
- * recover `{ pageId, relPath }`. Files without a parseable pageId in meta are
- * skipped (they are not engine-tracked pages — e.g. a stray hand-written file).
+ * Read every tracked .md file in the vault and recover `{ pageId, relPath }` from
+ * its `gitmost_id` frontmatter (native-Obsidian format). Files without a
+ * `gitmost_id` are skipped (they are not engine-tracked pages yet — e.g. a stray
+ * hand-written Obsidian file; PUSH adopts those separately).
  *
  * The IO is injected (R-Pull-1) so this is testable with fakes. Skip rules:
  *   - a `readFile` rejection (tracked but missing on disk, a mid-operation race)
  *     -> skipped, NOT thrown; the next pull converges;
- *   - unparseable meta (`parseDocmostMarkdown` throws) -> skipped;
- *   - parseable but no `pageId` in meta -> skipped.
+ *   - no `gitmost_id` frontmatter (`parsePageFile` -> id null) -> skipped.
  */
 export async function readExisting(
   deps: ReadExistingDeps,
@@ -111,15 +111,8 @@ export async function readExisting(
       // converges.
       continue;
     }
-    let pageId: string | undefined;
-    try {
-      const { meta } = parseDocmostMarkdown(text);
-      pageId = meta?.pageId;
-    } catch {
-      // Unparseable meta — not engine-tracked; leave it alone.
-      pageId = undefined;
-    }
-    if (pageId) existing.push({ pageId, relPath: rel });
+    const { id } = parsePageFile(text);
+    if (id) existing.push({ pageId: id, relPath: rel });
   }
   return existing;
 }
@@ -305,15 +298,13 @@ export async function applyPullActions(
   }): Promise<void> => {
     try {
       const page = await client.getPageJson(w.pageId);
-      const meta: PageMeta = {
-        version: 1,
-        pageId: page.id,
-        slugId: page.slugId,
-        title: page.title,
-        spaceId: page.spaceId,
-        parentPageId: page.parentPageId ?? null,
-      };
-      const text = await stabilizePageFile(page.content, meta);
+      // Native-Obsidian format: a minimal `gitmost_id` frontmatter + the fixpoint
+      // markdown body. title/parent/space are DERIVED (filename / folder / repo),
+      // so nothing but the pageId is persisted as meta.
+      const text = serializePageFile(
+        page.id,
+        await stabilizePageBody(page.content),
+      );
       const abs = relToAbs(vaultRoot, w.relPath);
       await deps.mkdir(dirname(abs));
       await deps.writeFile(abs, text);

packages/git-sync/src/engine/stabilize.ts

@@ -49,10 +49,30 @@ export async function stabilizePageFile(
   content: unknown,
   meta: PageMeta,
 ): Promise<string> {
-  const md1 = convertProseMirrorToMarkdown(content);
-  const doc2 = await markdownToProseMirror(md1);
-  const stableBody = convertProseMirrorToMarkdown(doc2);
   // The meta shape is exactly what `exportPageBody` writes; cast to the lib's
   // DocmostMdMeta (a superset with optional fields) for the serializer.
-  return serializeDocmostMarkdownBody(meta as DocmostMdMeta, stableBody);
+  return serializeDocmostMarkdownBody(
+    meta as DocmostMdMeta,
+    await stabilizePageBody(content),
+  );
+}
+
+/**
+ * The fixpoint markdown BODY for a page's ProseMirror `content`, WITHOUT any meta
+ * envelope:
+ *
+ *   md1        = convertProseMirrorToMarkdown(content)   // export...
+ *   doc2       = markdownToProseMirror(md1)              // ...import...
+ *   stableBody = convertProseMirrorToMarkdown(doc2)      // ...re-export
+ *
+ * The single export->import->export pass is the verified fixpoint (SPEC §11):
+ * idempotent for already-stable content, and the convergence point for the known
+ * converter asymmetries. The native-Obsidian writer (`serializePageFile`) wraps
+ * this body with a minimal `gitmost_id` frontmatter; determinism here is what
+ * keeps re-pulls of an unchanged page byte-identical (no churn, loop-guard).
+ */
+export async function stabilizePageBody(content: unknown): Promise<string> {
+  const md1 = convertProseMirrorToMarkdown(content);
+  const doc2 = await markdownToProseMirror(md1);
+  return convertProseMirrorToMarkdown(doc2);
 }