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

.gitignore

@@ -0,0 +1,17 @@
+# Dependencies
+node_modules/
+.pnpm-store/
+
+# Build output
+dist/
+coverage/
+
+# Local environment files (a committed .env.example lands in E00-S04)
+.env
+.env.*
+
+# Logs
+*.log
+
+# OS / editor
+.DS_Store