Use remote fs for turn diff repo root

[starr-openai]

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 #22202 Stabilize remote routing e2e tests

Summary:

• resolve the turn diff display root through the selected environment filesystem
• preserve local git-root lookup when no primary environment filesystem is present

Testing:

• Not run locally; relying on CI for this Codex PR.