Configurable working directory

[adrian-schmidt-foehre-mint]

Summary

• Adds a "Set Working Directory..." option to the settings menu that allows starting the terminal in a custom directory instead of the default solution directory
• Supports absolute paths (e.g. C:\Projects\MyRepo) and relative paths to the solution directory (e.g. ..\OtherRepo)
• Leave empty to use the default solution directory (fully backwards compatible)
• Setting is persisted in the existing JSON settings file and restored across sessions
• Changing the directory restarts the terminal automatically
• Real-time path validation in the input dialog - text turns red when the typed path doesn't exist

Files changed

• ClaudeCodeModels.cs - new CustomWorkingDirectory setting property
• ClaudeCodeControl.Workspace.cs - split directory resolution into base + override logic
• ClaudeCodeControl.ProviderManagement.cs - menu handler, WPF input dialog with live validation
• ClaudeCodeControl.xaml - new menu item

Test plan

• Open a solution, verify default behavior is unchanged (terminal starts in solution directory)
• Set an absolute path via the menu, verify terminal restarts in that directory
• Set a relative path (e.g. ..), verify it resolves correctly against the solution directory
• Type an invalid path in the dialog, verify text turns red
• Clear the setting, verify terminal returns to the default solution directory
• Press Cancel in the dialog, verify nothing changes
• Close and reopen VS, verify the setting persists