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/src/engine/push.ts

@@ -552,7 +552,18 @@ export async function applyPushActions(
   for (const u of actions.updates) {
     try {
       const fullMarkdown = await deps.readFile(u.path);
-      const result = await client.importPageMarkdown(u.pageId, fullMarkdown);
+      // The last-synced version of this file (pre-image) is the common ancestor
+      // for a 3-way merge against the live page, so concurrent human edits are
+      // not clobbered (review #5). Null when the file is new at last-pushed.
+      const baseMarkdown = await deps.git.showFileAtRef(
+        LAST_PUSHED_REF,
+        u.path,
+      );
+      const result = await client.importPageMarkdown(
+        u.pageId,
+        fullMarkdown,
+        baseMarkdown,
+      );
       updated++;
       // §10 loop-guard data: hash the body we pushed + capture `updatedAt`.
       pushed.push({