[codex] Tighten Windows sandbox read-root refresh

[bookholt-oai]

Summary

• Tighten the default Windows sandbox read-root refresh behavior.
• Preserve required helper, platform, workspace, and explicit override roots.
• Add regression coverage for the narrowed default refresh set.

Validation

• cargo test -p codex-windows-sandbox --lib
• Attempted cargo check -p codex-windows-sandbox --target x86_64-pc-windows-msvc; this Linux host lacks the MSVC lib.exe tool required by ring, so that target compile could not complete locally.

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