Add `agentseek plugin list` command

Summary

Add an agentseek plugin list command so users can inspect plugins installed in the AgentSeek plugin environment.

Requirements

• Add list under the existing agentseek plugin command group.
• Show the resolved plugin project path.
• List installed AgentSeek/Bub-related packages from the plugin sandbox, such as agentseek-* and bub-* dependencies.
• Support a human-readable default output.
• Do not install, update, remove, or sync packages as part of listing.

Acceptance Criteria

• Unit tests cover an empty plugin sandbox, a sandbox with installed plugin dependencies, and a missing sandbox path.
• Listing does not call the underlying uv helper or mutate the filesystem.
• Existing plugin install, uninstall, and update behavior remains unchanged.