Add script-based plugin system

[Niaobu]

Summary

• Adds a plugin system that lets users extend the CLI with custom commands backed by C# scripts executed via script.execute
• Plugins live in .unityctl/plugins/ (project-level, shareable via git) or ~/.unityctl/plugins/ (user-level, personal tools)
• Each plugin is a directory with a plugin.json manifest declaring commands, arguments, options, and script handlers
• The CLI dynamically creates System.CommandLine commands from discovered plugin manifests at startup
• Adds plugin list, plugin create <name>, and plugin remove <name> management commands
• Adds skill rebuild command and composes SKILL.md from three sources: base embedded skill + auto-generated plugin sections + user-provided .unityctl/skill-extra.md

Design

• No bridge or Unity plugin changes — plugin commands piggyback on existing script.execute RPC
• Project-level plugins take precedence over user-level plugins with the same name
• Plugins can provide a custom SKILL-SECTION.md for richer AI documentation, or get auto-generated docs from the manifest
• With no plugins and no skill-extra.md, behavior is identical to before

Test plan

• dotnet build succeeds
• All 261 existing tests pass
• plugin create scaffolds valid plugin with manifest + example script
• plugin list discovers and displays plugins from both sources
• Dynamic plugin commands appear in --help and accept declared args/options
• plugin remove cleans up plugin directory
• skill rebuild produces composed SKILL.md with plugin sections appended
• JSON output mode works for all new commands
• End-to-end: plugin command executes script in Unity via bridge