docs(git-sync): remove dangling references to the deleted git-sync-plan doc (PR #119 review)

The implementation spec docs/git-sync-plan.md was removed as completed, but ~44
code comments still cited it as "plan §N". Strip those citations (comments only),
keeping each comment grammatical. The vendored engine's own "SPEC §N" references
point at a different, still-present spec and are left untouched.

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

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

@@ -22,7 +22,7 @@
  * then calls `move_page` / `rename_page` (both for a reparent+retitle), or
  * records a NO-OP for a cosmetic local-only file-path rename.
  *
- * VENDORED into gitmost (plan §2.1/§3.1): the client seam is the native
+ * VENDORED into gitmost: the client seam is the native
  * `GitSyncClient` (`Pick<GitSyncClient, ...>`), not the upstream REST
  * `DocmostClient`; the upstream CLI `main()` entry point is dropped (the gitmost
  * server drives the engine in-process). Engine LOGIC is byte-identical.