feat: add unified `bundle list` command

[justinmerrell]

Summary

Add a bundle list command that shows both cached AND installed bundles in a unified view.

Functionality to implement

• List cached bundles (from cache store) with version and size info
• List installed bundles (from .musher/installed.json in current project) with harness and asset count
• Distinguish between cached-only and installed states
• Sort by recency
• Support --json output mode

Current state

musher-cli has cache list which shows cached bundles, but there is no unified view that also includes installation status for the current project.

Reference source (mush)

• cmd/mush/bundle.go — list subcommand
• internal/bundle/cache_index.go — ListCached(), ListCachedByRecency()
• internal/bundle/install.go — LoadInstalled(), FindInstalled()