fix(config): use XDG directory on new Linux installs

[y4ho0]

Summary

• Store config.json under $XDG_CONFIG_HOME/officecli on fresh Linux installs, falling back to ~/.config/officecli when the variable is unset or invalid.
• Preserve ~/.officecli/config.json for existing Linux installs and all non-Linux platforms.
• Keep an existing XDG config selected if logs, plugins, or caches later create ~/.officecli.
• Leave logs, plugins, caches, and the container /tmp fallback unchanged.
• Document the platform-specific config locations.

Closes #300

Validation

• dotnet run --project tests/OfficeCli.ConfigPath.Tests/OfficeCli.ConfigPath.Tests.csproj
• dotnet build src/officecli/officecli.csproj -c Release --nologo — 0 errors; one pre-existing CS8602 warning in ExcelHandler.SheetShift.cs:538
• Isolated macOS smoke test confirming config remains under ~/.officecli/config.json

[goworm]

Thanks for the careful implementation — the resolver and the tests are clean. We've decided not to move to XDG though (see #300): adoption is thinner than it looks, with Docker, npm, Cargo, rustup and ssh all still using their own home directory. And scoping it to config.json alone would split the config across two places, since plugins, caches and logs stay in ~/.officecli — either everything moves or nothing does. Closing.