Add optional worktree removal confirmation

[tim-smart]

Summary

• Add a client setting to control confirmation before removing an orphaned worktree.
• Allow immediate worktree removal when confirmation is disabled while preserving in-use worktrees.
• Expose the setting in General Settings with reset and restore-default support.
• Default confirmation to enabled for existing settings.

Testing

• Added unit coverage for worktree removal action selection.
• Added contract tests for the default value and settings patch decoding.
• Updated the desktop client settings fixture for the new field.
• Not run (test execution not provided).