Apps that use the default data directory share one %APPDATA%/Keybinding folder and overwrite each other's profiles #249

What's wrong

When no data directory is passed, every entry point falls back to the same path with nothing app-specific in it:

• Keybinding/Extensions/ServiceCollectionExtensions.cs:25 (AddKeybinding())
• Keybinding/Extensions/ServiceCollectionExtensions.cs:74 (AddKeybindingScoped())
• Keybinding/Services/KeybindingManagerFactory.cs:51 (CreateManager() fallback)
• Keybinding/Models/KeybindingConfiguration.cs:17

Each is Path.Combine(Environment.GetFolderPath(SpecialFolder.ApplicationData), "Keybinding"). That resolves to %APPDATA%\Keybinding on Windows and ~/.config/Keybinding on Linux for every consumer of the library. USAGE_EXAMPLES.md recommends calling AddKeybinding() with no directory.

Failure scenario

Two unrelated apps, A and B, both call services.AddKeybinding() and CreateDefaultProfile(), which uses id "default".

1. A binds editor.save to Ctrl+S and saves.
2. B binds game.jump to Space and saves.
   • JsonKeybindingRepository.SaveProfileAsync runs RemoveAll(p => p.Id == profile.Id), so B's profile replaces A's "default" profile and all of A's bindings.
   • B also writes its own active profile to the shared active-profile.json.
   • The command merge in KeybindingManager keeps stored commands it did not load, so A's commands stay in the shared commands.json.
3. A restarts and calls InitializeAsync:
   • Ctrl+S is unbound, and the "default" profile holds game.jump on Space.
   • B's commands appear in A's GetAllCommands, SearchCommands and GetSummary().TotalCommands.

The user's bindings are lost and nothing reports it.

Suggested fix / acceptance criteria

• Define the default directory once, for example in Constants, and use that definition at all four call sites.
• Make the default app-specific. For example, Path.Combine(ApplicationData, Assembly.GetEntryAssembly()?.GetName().Name ?? "Keybinding", "Keybinding"), or an appName parameter on AddKeybinding/CreateManager. Making the directory required is the other option.
• Document the default location in the README.
• Add a test: two configurations with different entry or app names resolve to different directories, and the default path contains the app name.
• Optional: when the old shared folder exists, log it or migrate from it once, so existing single-app users keep their bindings.

Related but different: #93 proposes moving persistence onto ktsu.AppDataStorage, which would also scope storage per app. This issue is the narrower data-loss bug in the current default.