feat: add root build/test/typecheck scripts (E00-S01-T05)

Adds root scripts so build, test and typecheck run from the workspace root:
- root package.json gains scripts: build (pnpm -r run build), test
  (node --test on tests/**/*.test.mjs), typecheck (pnpm -r run typecheck)
- every workspace package (apps/server, packages/core,
  extensions/example) gains build (tsc -p tsconfig.json) and typecheck
  (tsc -p tsconfig.json --noEmit) scripts
- typescript 6.0.3 pinned as an exact root devDependency so the commands
  run from a clean checkout; lockfile regenerated with pnpm 11.23.0

Verified from a clean state: pnpm install --frozen-lockfile passes,
pnpm build emits dist for all 3 packages, pnpm typecheck passes for all 3,
pnpm test runs tests/architecture-import.test.mjs 10/10 green, and CI's
pnpm -r list --depth -1 still lists all 4 workspace projects.

Closes #158

packages/core/package.json

@@ -4,6 +4,10 @@
   "private": true,
   "type": "module",
   "description": "EPPP application core. Bootstrap placeholder — owns application lifecycle, site identity and content primitives in later stories.",
+  "scripts": {
+    "build": "tsc -p tsconfig.json",
+    "typecheck": "tsc -p tsconfig.json --noEmit"
+  },
   "main": "./dist/index.js",
   "types": "./dist/index.d.ts",
   "exports": {