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>

tsconfig.json

@@ -6,5 +6,6 @@
       "@/*": ["./src/*"]
     }
   },
-  "include": ["**/*.ts", "**/*.tsx", ".expo/types/**/*.ts", "expo-env.d.ts"]
+  "include": ["**/*.ts", "**/*.tsx", ".expo/types/**/*.ts", "expo-env.d.ts"],
+  "exclude": ["**/*.test.ts", "**/*.test.tsx"]
 }