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.

bun.lock

@@ -4,11 +4,6 @@
   "workspaces": {
     "": {
       "name": "code",
-      "dependencies": {
-        "@code/env": "workspace:*",
-        "dotenv": "catalog:",
-        "zod": "catalog:",
-      },
       "devDependencies": {
         "@code/config": "workspace:*",
         "@types/bun": "latest",
@@ -67,7 +62,6 @@
         "@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",
@@ -130,7 +124,6 @@
         "@tanstack/react-form": "catalog:",
         "better-auth": "catalog:",
         "convex": "catalog:",
-        "dotenv": "catalog:",
         "isbot": "^5.1.44",
         "lucide-react": "catalog:",
         "next-themes": "catalog:",
@@ -158,7 +151,10 @@
       "name": "@code/agents",
       "version": "0.0.0",
       "dependencies": {
+        "@code/env": "workspace:*",
         "@flue/runtime": "latest",
+        "convex": "catalog:",
+        "hono": "^4.12.30",
       },
       "devDependencies": {
         "@code/config": "workspace:*",
@@ -171,10 +167,10 @@
       "version": "1.0.0",
       "dependencies": {
         "@better-auth/expo": "catalog:",
+        "@code/env": "workspace:*",
         "@convex-dev/better-auth": "catalog:",
         "better-auth": "catalog:",
         "convex": "catalog:",
-        "dotenv": "catalog:",
         "zod": "catalog:",
       },
       "devDependencies": {
@@ -192,7 +188,6 @@
       "version": "0.0.0",
       "dependencies": {
         "@t3-oss/env-core": "^0.13.11",
-        "dotenv": "catalog:",
         "zod": "catalog:",
       },
       "devDependencies": {