refactor(sessions): single source of truth for session directory scope (#10)

Extract the list/create scope rule into sessionScopeDirectory() so loadSessions
and createSession can no longer drift apart — the root cause of #10 (sessions
empty after connect/create). Add a zero-dependency node:test regression guard
proving both paths resolve identically across all inputs, a 'test' npm script,
and exclude test files from the app typecheck.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

package.json

@@ -8,7 +8,8 @@
     "start:clear": "expo start --clear",
     "ios": "expo run:ios",
     "android": "expo run:android",
-    "typecheck": "tsc --noEmit"
+    "typecheck": "tsc --noEmit",
+    "test": "node --test 'src/**/*.test.ts'"
   },
   "dependencies": {
     "@expo/vector-icons": "^15.0.3",