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.

.gitignore

@@ -11,9 +11,10 @@ build
 # Generated files
 apps/web/src/routeTree.gen.ts
 
-# Environment variables
+# Environment variables live at the repository root
 .env
 .env*.local
+!.env.example
 
 # IDEs and editors
 .vscode/*