feat(git-sync): three-way body merge using the last-synced base (no edit loss)

Upgrades the 2-way body merge to a real diff3 three-way merge (review #5), so a
block ONLY the human changed is KEPT when git changed a DIFFERENT block — the
2-way merge would revert it to git's stale version.

Engine: the push update loop reads the last-synced pre-image
(`git.showFileAtRef(refs/docmost/last-pushed, path)`) and passes it as the
optional `baseMarkdown` to `client.importPageMarkdown` (the common ancestor).

Server: gitmost-datasource converts base+incoming, and writeBody runs a block-
level diff3 (new three-way-merge.ts `diff3Plan`): live-only change -> keep live,
git-only change -> take git, both-changed -> git wins (conflict policy), inserts/
deletes from either side preserved. Without a base (createPage) it falls back to
the 2-way merge. Crash-safety unchanged (docs built before the connection opens).

Tests: three-way-merge.spec.ts (14 — every diff3 case incl. the cross-block
preservation and conflict policy), yjs-body-merge 3-way (real Y.Docs: human's
block instance preserved while git's block is applied), plus an engine test that
the base is forwarded from showFileAtRef. Existing push assertions updated for the
new base arg. git-sync 589 pass; server merge/datasource/gate 62 pass; typecheck
clean.

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

packages/git-sync/test/apply-push-actions.test.ts

@@ -143,12 +143,36 @@ describe('applyPushActions — update (collab path, SPEC §2/§15.6)', () => {
     expect(res.updated).toBe(1);
     // The collab/Yjs write path is used — NOT a raw jsonb overwrite.
     expect(client.importPageMarkdown).toHaveBeenCalledTimes(1);
-    expect(client.importPageMarkdown).toHaveBeenCalledWith('p-1', fileBody);
+    expect(client.importPageMarkdown).toHaveBeenCalledWith('p-1', fileBody, null);
     // No raw-overwrite path exists on the injected client surface at all.
     expect((client as any).updatePageJson).toBeUndefined();
     expect(client.createPage).not.toHaveBeenCalled();
     expect(client.deletePage).not.toHaveBeenCalled();
   });
+
+  it('forwards the last-pushed base body (3-way merge ancestor) when present', async () => {
+    const baseBody =
+      '<!-- docmost:meta\n{"version":1,"pageId":"p-1"}\n-->\n\nbase body\n';
+    const fileBody =
+      '<!-- docmost:meta\n{"version":1,"pageId":"p-1"}\n-->\n\nupdated body\n';
+    const client = makeClient();
+    // The pre-image (refs/docmost/last-pushed) carries the base version.
+    const { git } = makeGit({ prevTree: { 'Doc.md': baseBody } });
+    const fs = makeFs({ 'Doc.md': fileBody });
+
+    await applyPushActions(
+      deps(client, git, fs),
+      actions({ updates: [{ pageId: 'p-1', path: 'Doc.md' }] }),
+    );
+
+    // importPageMarkdown receives the base so the server can 3-way merge it.
+    expect(client.importPageMarkdown).toHaveBeenCalledWith(
+      'p-1',
+      fileBody,
+      baseBody,
+    );
+    expect(git.showFileAtRef).toHaveBeenCalledWith(LAST_PUSHED_REF, 'Doc.md');
+  });
 });
 
 describe('applyPushActions — create (assigned pageId written back to meta)', () => {
@@ -542,8 +566,8 @@ describe('applyPushActions — per-page error isolation + refs gated on success
     // The 1st and 3rd were applied; the 2nd threw.
     expect(res.updated).toBe(2);
     expect(client.importPageMarkdown).toHaveBeenCalledTimes(3);
-    expect(client.importPageMarkdown).toHaveBeenNthCalledWith(1, 'p-a', 'a body');
-    expect(client.importPageMarkdown).toHaveBeenNthCalledWith(3, 'p-c', 'c body');
+    expect(client.importPageMarkdown).toHaveBeenNthCalledWith(1, 'p-a', 'a body', null);
+    expect(client.importPageMarkdown).toHaveBeenNthCalledWith(3, 'p-c', 'c body', null);
 
     // The failure is recorded with kind/pageId/path/error.
     expect(res.failures).toEqual([
@@ -649,7 +673,7 @@ describe('applyPushActions — mixed batch + skipped passthrough', () => {
     expect(res.writtenBack).toEqual([{ path: 'N.md', pageId: 'created-1' }]);
     expect(res.skipped).toEqual(skipped);
     expect(updateRefCalls).toEqual([{ ref: LAST_PUSHED_REF, target: 'sha-9' }]);
-    expect(client.importPageMarkdown).toHaveBeenCalledWith('u-1', updFile);
+    expect(client.importPageMarkdown).toHaveBeenCalledWith('u-1', updFile, null);
     expect(client.deletePage).toHaveBeenCalledWith('d-1');
   });
 });

packages/git-sync/test/engine-gaps.test.ts

@@ -300,7 +300,11 @@ describe('applyPushActions (push.ts) — move prefetch isolation', () => {
     // The update and the delete in the SAME batch still applied.
     expect(res.updated).toBe(1);
     expect(res.deleted).toBe(1);
-    expect(client.importPageMarkdown).toHaveBeenCalledWith('u1', store['Up.md']);
+    expect(client.importPageMarkdown).toHaveBeenCalledWith(
+      'u1',
+      store['Up.md'],
+      null,
+    );
     expect(client.deletePage).toHaveBeenCalledWith('d1');
 
     // The broken move was ISOLATED: no movePage/renamePage call, recorded as a

packages/git-sync/test/run-push.test.ts

@@ -312,7 +312,7 @@ describe('runPush — --apply is the ONLY write path', () => {
 
     const res = await runPush(deps, { dryRun: false });
 
-    expect(client.importPageMarkdown).toHaveBeenCalledWith('p-9', file);
+    expect(client.importPageMarkdown).toHaveBeenCalledWith('p-9', file, null);
     expect(res.applied?.updated).toBe(1);
   });
 });