Stabilize remote routing e2e tests

[starr-openai]

Why

The remote routing e2e coverage needs a harness where the remote environment is the default execution target but local is still explicitly selectable. Without that shape, routing tests can pass or fail based on environment ordering instead of the environment_id contract they are meant to verify.

What changed

• add a test-only remote-aware environment manager that keeps local addressable alongside a remote default
• let the common Codex test builder opt into that remote-and-local harness
• move the affected remote routing tests onto the explicit mixed-environment fixture
• clean up the remote image fixture so the test exercises routing rather than decoder fragility

Stack

• PR #22199 Isolate tmp-dependent tests from ambient git
• PR #22201 Use remote fs for turn diff repo root
• PR #22200 Emit unified exec end on startup failure
• PR #22389 Stabilize remote routing e2e tests
• PR #22540 Make proxy decider test DNS independent

Validation

• Current-head CI is green.
• No local Codex test run; relying on CI for this stack.