feat: configure ESM package boundaries across the workspace (E00-S01-T03)

Adds ESM boundary declarations to every workspace package manifest
(apps/server, packages/core, extensions/example): "type": "module",
"main"/"types" entry points and an "exports" map (types + import
conditions) so each package exposes only its public root; adds
"type": "module" to the workspace root package.json so the workspace is
uniformly ESM. Placeholder src/index.ts files stay as empty ESM modules.

Verified: each package compiles under tsconfig.base.json (NodeNext) with
TypeScript 6.0.3 and emits ESM dist/index.js + dist/index.d.ts; Node
imports each package by name through its exports map and rejects deep
subpath imports (ERR_PACKAGE_PATH_NOT_EXPORTED); pnpm 11.23.0
install --frozen-lockfile passes with the lockfile unchanged.

Closes #156

extensions/example/package.json

@@ -2,5 +2,14 @@
   "name": "@personal-blog/example-extension",
   "version": "0.0.0",
   "private": true,
-  "description": "Placeholder extension that exercises the extensions/ workspace group. Real extensions (e.g. theme-amber) replace it in later stories."
+  "type": "module",
+  "description": "Placeholder extension that exercises the extensions/ workspace group. Real extensions (e.g. theme-amber) replace it in later stories.",
+  "main": "./dist/index.js",
+  "types": "./dist/index.d.ts",
+  "exports": {
+    ".": {
+      "types": "./dist/index.d.ts",
+      "import": "./dist/index.js"
+    }
+  }
 }