core: add provider test coverage for upcoming refactor

Add comprehensive test suite for Provider module to ensure safe
refactoring of provider internals. Tests cover:
- Provider loading from env vars and config
- Provider filtering (disabled_providers, enabled_providers)
- Model whitelist/blacklist
- Model aliasing and custom providers
- getModel, getProvider, closest, defaultModel functions

Also adds Env module for instance-scoped environment variable access,
enabling isolated test environments without global state pollution.

packages/opencode/src/env/index.ts

@@ -0,0 +1,26 @@
+import { Instance } from "../project/instance"
+
+export namespace Env {
+  const state = Instance.state(() => {
+    return { ...process.env } as Record<string, string | undefined>
+  })
+
+  export function get(key: string) {
+    const env = state()
+    return env[key]
+  }
+
+  export function all() {
+    return state()
+  }
+
+  export function set(key: string, value: string) {
+    const env = state()
+    env[key] = value
+  }
+
+  export function remove(key: string) {
+    const env = state()
+    delete env[key]
+  }
+}