feat: T1 scaffold — package.json, tsconfig (backend+web), vitest, dirs

- All deps declared once (express5, ws, node-pty, @xterm/xterm6, addon-fit;
  dev: typescript6, tsx, vitest4, @types/node|ws|express)
- postinstall chmod +x node-pty spawn-helper (prebuild ships it non-exec → posix_spawnp fails)
- node-pty verified: require + real PTY spawn (exitCode 0)
- npm test passes with no tests (passWithNoTests)
- tsconfig.web.json type-checks frontend; browser bundling strategy still TBD (see log)

T1 of docs/PLAN.md

vitest.config.ts

@@ -0,0 +1,15 @@
+import { defineConfig } from 'vitest/config'
+
+export default defineConfig({
+  test: {
+    include: ['test/**/*.test.ts'],
+    environment: 'node',
+    // Scaffold has no tests yet; don't fail the script until modules add theirs.
+    passWithNoTests: true,
+    // Coverage target is the global 80% rule; enable with `--coverage` once modules exist.
+    coverage: {
+      provider: 'v8',
+      include: ['src/**/*.ts', 'public/**/*.ts'],
+    },
+  },
+})