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.

packages/env/package.json

@@ -4,13 +4,14 @@
   "private": true,
   "type": "module",
   "exports": {
-    "./web": "./src/web.ts",
+    "./agent": "./src/agent.ts",
+    "./convex": "./src/convex.ts",
     "./native": "./src/native.ts",
-    "./server": "./src/server.ts"
+    "./server": "./src/server.ts",
+    "./web": "./src/web.ts"
   },
   "dependencies": {
     "@t3-oss/env-core": "^0.13.11",
-    "dotenv": "catalog:",
     "zod": "catalog:"
   },
   "devDependencies": {