chore(release): exit changesets pre-release (RC) mode for stable 16.0.0

Flips `.changeset/pre.json` mode `pre` -> `exit` (equivalent to
`changeset pre exit`). The next `changeset version` on main will roll every
changeset accumulated during the RC window into a single stable bump — the
whole lockstep `fixed` group goes 15.1.1 -> 16.0.0 (driven by the `major`
changeset removing the deprecated `session.tenantId` alias) — dropping the
`-rc.N` suffix and publishing `latest` instead of the `rc` dist-tag.

No changeset added (this is a release-process flip, not a package change):
carries the `skip-changeset` label, which also stands the re-armed no-major
guard down for this PR — the whole-stack major is intentional here.

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

Author: os-zhuang

.changeset/pre.json

@@ -1,5 +1,5 @@
 {
-  "mode": "pre",
+  "mode": "exit",
   "tag": "rc",
   "initialVersions": {
     "@objectstack/docs": "4.2.1",