Sets io.IniFilename to %APPDATA%\UsdLayerManager\imgui.ini right after
CreateContext so ImGui never touches the working directory for layout
persistence. The directory is created on first run via create_directories.
m_iniFilePath on ImGuiContext holds the string so the pointer stays valid
for the lifetime of the context.
Also removes imgui.ini from version control and adds it to .gitignore.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
indigo