feat(core,tooling): Windows support — shell selection, install.ps1, win-x64 release package, Windows CI (#79)

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

packages/core/vitest.config.ts

@@ -0,0 +1,16 @@
+/**
+ * Vitest config for core. The one non-default setting is a platform-aware test timeout:
+ * many core tests spawn a real shell (Git-Bash on the Windows CI runners) or write the
+ * full Agent State layout, and Windows runner I/O variance (cold shell spawns, Defender
+ * first-touch scans, slow-disk moments) has pushed individually fast tests past vitest's
+ * 5s default — a different test on each run. A larger failure deadline changes nothing
+ * for passing tests; POSIX keeps the 5s default to fail fast during local development.
+ */
+import { defineConfig } from "vitest/config";
+
+export default defineConfig({
+  test: {
+    environment: "node",
+    testTimeout: process.platform === "win32" ? 30_000 : 5_000,
+  },
+});