feat: add `bundle uninstall` command

[justinmerrell]

Summary

Add a bundle uninstall command that removes previously installed bundle assets from the project directory.

Functionality to implement

• Read installation tracking from .musher/installed.json
• Remove all files listed in the installation entry's installedAssets
• Interactive confirmation prompt before deletion
• --force flag to skip confirmation
• Clean up empty directories after asset removal
• Update the installation registry to remove the entry

Reference source (mush)

• cmd/mush/bundle.go — uninstall subcommand wiring
• internal/bundle/install.go — Uninstall() function, FindInstalled()

Context

The install registry infrastructure exists (internal/bundle/install/registry.go with Track()) but there is no CLI command to reverse an installation.