Avoid whole plugin cache replacement on Windows

[zm-oai]

Summary

• keep the existing whole-root atomic plugin-cache replacement path for non-Windows
• on Windows, activate installs at the plugin version directory instead of renaming the whole plugin cache root
• clean up stale plugin versions best-effort after the new version is active
• add Windows regression tests that hold cwd inside the existing plugin cache root while reinstalling/upgrading

Testing

• cargo test -p codex-core-plugins --lib store::tests::windows_install
• cargo clippy --tests --no-deps -p codex-core-plugins -- -D warnings
• cargo fmt -p codex-core-plugins
• git diff --check

Also ran cargo test -p codex-core-plugins --lib; the new Windows tests passed, but the full package run hit 4 unrelated Git-for-Windows/MSYS local-clone failures with CreateFileMapping / signal-pipe Win32 error 5.

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