feat: add `bundle install` command

[justinmerrell]

Summary

Add a bundle install command that installs bundle assets into the current project's harness-native directory structure.

Functionality to implement

• Map assets to harness-native directories (e.g. .claude/skills/, .claude/agents/, .mcp.json)
• Conflict detection with InstallConflictError when files already exist
• JSON/TOML deep merge for tool config files (not overwrite)
• --force flag to overwrite existing files
• --harness flag to select target harness
• Track installations in .musher/installed.json (registry infrastructure already exists in internal/bundle/install/)

Reference source (mush)

• cmd/mush/bundle_install.go — CLI command and orchestration
• internal/bundle/materialize.go — InstallFromCache(), conflict handling
• internal/bundle/merge.go — MergeJSONDocs(), MergeTOMLDocs()
• internal/bundle/mapper.go — AssetMapper interface for harness-native paths

Context

musher-cli already has the install registry tracking (internal/bundle/install/registry.go) and session materialization (internal/bundle/session/), but no CLI command to drive project-level installation.