style(miles): move radix os import to module scope

[TianyeGGBond]

Context

This is a small style cleanup for the RLix guard in RadixTreeMiddleware.

The code had a local import:

import os as _os  # local import ? module top imports must change

That comment is self-contradictory: it says the import belongs at module scope while keeping it local. There is also no practical reason to delay-import os; it is a standard-library module, and the nearby RLix-mode call sites in router.py and multi_turn.py already use top-level import os.

Change

• Move os to the module import block.
• Use os.environ directly in the RLix-mode guard.
• Keep behavior unchanged: RLIX_CONTROL_PLANE=rlix still rejects RadixTreeMiddleware initialization.

Validation

Passed:

python -m compileall miles/router/middleware_hub/radix_tree_middleware.py
git diff --check origin/zhenyu/m11-mvp-test..HEAD

Also ran a focused initialization check with temporary local import stubs for heavy optional dependencies missing in this Windows environment. With RLIX_CONTROL_PLANE=rlix, RadixTreeMiddleware(...) still raises the expected RuntimeError.