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.

tsconfig.base.json

@@ -0,0 +1,20 @@
+{
+  "compilerOptions": {
+    "target": "ES2023",
+    "lib": ["ES2023"],
+    "module": "NodeNext",
+    "moduleResolution": "NodeNext",
+    "strict": true,
+    "noUncheckedIndexedAccess": true,
+    "exactOptionalPropertyTypes": true,
+    "noImplicitOverride": true,
+    "useUnknownInCatchVariables": true,
+    "verbatimModuleSyntax": true,
+    "isolatedModules": true,
+    "esModuleInterop": true,
+    "forceConsistentCasingInFileNames": true,
+    "declaration": true,
+    "sourceMap": true,
+    "skipLibCheck": true
+  }
+}