feat: bootstrap pnpm workspace (E00-S01)

pnpm 11.23.0 workspace with a committed frozen lockfile, Node engine
restricted to the 24.x line, TypeScript 6.0.3 pinned exactly, strict base
tsconfig, apps/packages/extensions kept as separate workspaces, root
build/test/typecheck commands, and the FIT-001 architecture test rejecting
core-to-extension imports. Minimal CI runs only the root scripts (S05
stages stay out of scope). Closes #58.

extensions/example/package.json

@@ -0,0 +1,22 @@
+{
+  "name": "@personal-blog/example-extension",
+  "version": "0.0.0",
+  "private": true,
+  "type": "module",
+  "description": "Placeholder extension that exercises the extensions/ workspace group and the FIT-001 dependency boundary. 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",
+      "default": "./dist/index.js"
+    }
+  },
+  "files": [
+    "dist"
+  ],
+  "scripts": {
+    "build": "tsc -p tsconfig.json",
+    "typecheck": "tsc -p tsconfig.json --noEmit"
+  }
+}

extensions/example/src/index.ts

@@ -0,0 +1,7 @@
+/**
+ * @personal-blog/example-extension — bootstrap placeholder extension.
+ *
+ * FIT-001: core packages (packages/*) must never import a concrete extension
+ * like this one. Enforced by tests/architecture.test.ts in CI.
+ */
+export const EXAMPLE_EXTENSION_ID = 'example';

extensions/example/tsconfig.json

@@ -0,0 +1,8 @@
+{
+  "extends": "../../tsconfig.base.json",
+  "compilerOptions": {
+    "rootDir": "src",
+    "outDir": "dist"
+  },
+  "include": ["src"]
+}