AppContext.BaseDirectory pointed to the runtime/ subfolder where the
self-contained .NET payload lives, causing plugins that use it for file
paths to write to the wrong directory. Now set to the server exe
directory at startup via AppContext.SetData.
Also adds serverDirectory and dataDirectory properties to ServerPlugin
so plugin authors have convenient access to the server root and a
per-plugin data folder (plugins/<PluginName>/) without needing to
resolve paths manually.
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