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/web/package.json

@@ -19,7 +19,6 @@
     "@tanstack/react-form": "catalog:",
     "better-auth": "catalog:",
     "convex": "catalog:",
-    "dotenv": "catalog:",
     "isbot": "^5.1.44",
     "lucide-react": "catalog:",
     "next-themes": "catalog:",