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

pnpm-lock.yaml

@@ -0,0 +1,15 @@
+lockfileVersion: '9.0'
+
+settings:
+  autoInstallPeers: true
+  excludeLinksFromLockfile: false
+
+importers:
+
+  .: {}
+
+  apps/server: {}
+
+  extensions/example: {}
+
+  packages/core: {}