Consolidate env at repo root

- Move all application env vars to repository-root .env (with .env.example
  as the committed template) and drop the per-app .env.example files.
- Add @code/env subpaths (agent, convex) and consume validated exports
  everywhere instead of reading process.env directly.
- Wire every dev/build/start script to load ../../.env via bun --env-file
  and point vite.config.ts envDir at the repo root.

apps/native/package.json

@@ -4,12 +4,12 @@
   "private": true,
   "main": "expo-router/entry",
   "scripts": {
-    "start": "expo start",
-    "dev": "expo start --clear",
-    "android": "expo run:android",
-    "ios": "expo run:ios",
-    "prebuild": "expo prebuild",
-    "web": "expo start --web"
+    "start": "bun --env-file=../../.env expo start",
+    "dev": "bun --env-file=../../.env expo start --clear",
+    "android": "bun --env-file=../../.env expo run:android",
+    "ios": "bun --env-file=../../.env expo run:ios",
+    "prebuild": "bun --env-file=../../.env expo prebuild",
+    "web": "bun --env-file=../../.env expo start --web"
   },
   "dependencies": {
     "@better-auth/expo": "catalog:",
@@ -22,7 +22,6 @@
     "@tanstack/react-form": "catalog:",
     "better-auth": "catalog:",
     "convex": "catalog:",
-    "dotenv": "catalog:",
     "expo": "~57.0.1",
     "expo-constants": "~57.0.2",
     "expo-font": "~57.0.0",