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/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