build(git-sync): land the @docmost/git-sync package into develop, code-only (#326 step 1 / PR-A)

The git-sync converter + engine source lived only on the #119 branch; develop
had just the dead compiled build/. Bring the whole package (src + ~700 tests)
onto develop under CI, with NO consumer wired — git-sync stays fully inert in
develop (nothing in apps/server imports it), so runtime behavior is unchanged.
This unblocks #293 (extract the shared converter package from the landed source)
and lets #119's functionality land LAST, already writing the canonical format
(per the #326 landing order).

- packages/git-sync: src (lib converter + engine) + test corpus + configs.
- Remove develop's dead committed packages/git-sync/build/; gitignore it
  (built in CI/Docker via pnpm build, never committed — no src/build drift).
- pnpm-lock.yaml: add the @docmost/git-sync importer (a missing workspace
  package in the lock is a CI blocker). `pnpm install --frozen-lockfile` passes.
- NO server integration / loader / Dockerfile runtime changes (those come with
  #119 at step 6).

Verified: tsc clean; vitest 711 passed | 1 expected-fail, 0 failures, 0 type
errors; pnpm --frozen-lockfile EXIT 0; apps/server has no git-sync import.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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

@@ -0,0 +1,78 @@
+/**
+ * Normalize-on-write helper (SPEC §11 "Resolution").
+ *
+ * git diffs byte-for-byte, so writing a page in a NON-fixpoint markdown form
+ * would make the next pull re-export it to a slightly different (but stable)
+ * form and produce a phantom diff -> churny commits. The converter has a couple
+ * of known one-pass asymmetries (a block image after a paragraph adds an empty
+ * paragraph; a diagram materializes `data-align`), all of which converge to a
+ * fixpoint after ONE `export -> import -> export` round-trip.
+ *
+ * So at write time we run exactly that one pass and persist the fixpoint form.
+ * Already-stable content is unaffected (the pass is idempotent), so re-pulls of
+ * unchanged pages produce identical bytes and git sees no diff.
+ */
+import {
+  convertProseMirrorToMarkdown,
+  markdownToProseMirror,
+  serializeDocmostMarkdownBody,
+  type DocmostMdMeta,
+} from "../lib/index.js";
+
+/**
+ * Meta object as `exportPageBody` builds it (SPEC §4). Kept byte-for-byte
+ * compatible so files produced here match `exportPageBody`'s output exactly.
+ */
+export interface PageMeta {
+  version: 1;
+  pageId: string;
+  slugId: string;
+  title: string;
+  spaceId: string;
+  parentPageId: string | null;
+}
+
+/**
+ * Produce the self-contained `.md` file text for a page from its raw
+ * ProseMirror `content` + identity meta, in the verified fixpoint form.
+ *
+ *   md1        = convertProseMirrorToMarkdown(content)
+ *   doc2       = markdownToProseMirror(md1)            // one import...
+ *   stableBody = convertProseMirrorToMarkdown(doc2)    // ...and re-export
+ *   file       = serializeDocmostMarkdownBody(meta, stableBody)
+ *
+ * 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.
+ */
+export async function stabilizePageFile(
+  content: unknown,
+  meta: PageMeta,
+): Promise<string> {
+  // 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,
+    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);
+}