fix(installer): preserve credentials dir and seed STANDARDS.md on install

- Add credentials/ to PRESERVE_PATHS so rsync --delete doesn't wipe
  user credential stores on framework upgrades
- Add AGENTS.md and STANDARDS.md to PRESERVE_PATHS to protect
  user-customized copies from being deleted during sync
- Seed AGENTS.md and STANDARDS.md from defaults/ on first install
  (previously only existed in defaults/ but never copied to framework root)
- Create credentials/ directory during install (alongside memory/)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/mosaic/package.json

@@ -1,6 +1,6 @@
 {
   "name": "@mosaicstack/mosaic",
-  "version": "0.0.29",
+  "version": "0.0.28",
   "repository": {
     "type": "git",
     "url": "https://git.mosaicstack.dev/mosaicstack/stack.git",