docs(dirs): note that XDG_* is ignored on macOS/Windows and must stay that way

[ErikBjare]

Comment-only. platformdirs (and the dirs crate in aw-server-rust) ignore XDG_* on macOS/Windows, and all components must resolve identical paths. Documents this so nobody special-cases XDG_* in one component (see ActivityWatch/aw-client#119).

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[greptile-apps]

Confidence Score: 5/5

[Low risk] Adds a comment to the directory resolution code.

The PR appears safe to merge.

Summary

This comment-only PR documents platformdirs’ platform-specific XDG behavior and advises against component-specific XDG path handling.

• It also points tests toward patching platformdirs, consistent with the existing directory tests.

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