chore: bootstrap pnpm workspace and package manifests

E00-S01-T01: create workspace/package manifests
- root package.json pins packageManager pnpm@11.23.0
- pnpm-workspace.yaml separates apps/*, packages/*, extensions/* entries
- skeleton manifests for apps/server, packages/core, extensions/example
- committed frozen lockfile generated by pnpm 11.23.0
- minimal CI: pnpm install --frozen-lockfile + workspace verification

package.json

@@ -0,0 +1,7 @@
+{
+  "name": "personal-blog",
+  "version": "0.0.0",
+  "private": true,
+  "description": "EPPP — a modular personal blogging platform. Workspace bootstrap (E00-S01).",
+  "packageManager": "pnpm@11.23.0"
+}