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.

.env.example

@@ -0,0 +1,32 @@
+# Shared application endpoints
+CONVEX_DEPLOYMENT=dev:example-deployment
+CONVEX_URL=https://example.convex.cloud
+CONVEX_SITE_URL=https://example.convex.site
+SITE_URL=http://localhost:5173
+NATIVE_APP_URL=code://
+
+# Browser and native public endpoints
+VITE_CONVEX_URL=https://example.convex.cloud
+VITE_CONVEX_SITE_URL=https://example.convex.site
+EXPO_PUBLIC_CONVEX_URL=https://example.convex.cloud
+EXPO_PUBLIC_CONVEX_SITE_URL=https://example.convex.site
+
+# Daemon identity and control plane
+DAEMON_ID=local-macbook
+DAEMON_NAME=Local MacBook
+DAEMON_VERSION=0.0.0
+DAEMON_HEARTBEAT_MS=15000
+DAEMON_COMMAND_LEASE_MS=60000
+# RIVET_ENDPOINT=http://localhost:6420
+
+# Flue persistence adapter
+FLUE_DB_TOKEN=replace-with-a-long-random-token
+
+# Agent model provider
+AGENT_MODEL_PROVIDER=cheaptricks
+AGENT_MODEL_NAME=minimax-m3
+AGENT_MODEL_API=openai-completions
+AGENT_MODEL_BASE_URL=https://ai.example.com/v1
+AGENT_MODEL_API_KEY=replace-with-provider-api-key
+AGENT_MODEL_CONTEXT_WINDOW=262000
+AGENT_MODEL_MAX_TOKENS=131072