Add plugin system (script + executable)

[Niaobu]

Summary

Adds a plugin system that lets users extend unityctl with custom commands — no changes to the bridge or Unity plugin required.

Two plugin types:

• Script plugins — C# scripts executed inside Unity via the existing script.execute RPC. Best for commands that need Unity APIs (scene queries, asset manipulation, etc.).
• Executable plugins — any unityctl-<name> binary/script on disk, following the git/kubectl naming convention. Best for orchestration, CI pipelines, and workflows outside Unity.

How it works

Script plugins

A plugin.json manifest declares commands with typed arguments, options, and a C# handler script. The handler runs on Unity's main thread with full access to UnityEngine/UnityEditor APIs.

.unityctl/plugins/scene-stats/
  plugin.json       # manifest: commands, args, options, handler mapping
  stats.cs          # C# script executed in Unity
  SKILL-SECTION.md  # optional custom AI documentation

# Scaffold a new plugin
unityctl plugin create scene-stats

# Use it (dispatches to Unity via bridge)
unityctl scene-stats stats --verbose
unityctl scene-stats stats MainScene --format json

Executable plugins

Place any executable named unityctl-<name> in .unityctl/plugins/ or on PATH. All arguments are passed through; bridge connection info is injected via environment variables (UNITYCTL_BRIDGE_URL, UNITYCTL_PROJECT_PATH, etc.).

# A bash script saved as .unityctl/plugins/unityctl-smoke
unityctl smoke 30          # runs unityctl-smoke with arg "30"
unityctl deploy --staging  # runs unityctl-deploy with arg "--staging"

Executables in plugin directories appear in --help at startup. PATH executables are resolved lazily at invocation time (git-style fallback).

Plugin locations

Level	Directory	Precedence
Project	.unityctl/plugins/	Higher (shareable via git)
User	~/.unityctl/plugins/	Lower (personal tools)

Skill composition

Plugins can provide AI documentation via custom SKILL-SECTION.md files or auto-generated docs from the manifest. The new skill rebuild command composes the final SKILL.md from:

1. Base embedded skill (source of truth in UnityCtl.Cli/Resources/SKILL.md)
2. Auto-generated plugin command sections
3. Optional user-provided .unityctl/skill-extra.md

Design decisions

• Zero bridge/Unity changes — script plugins reuse script.execute RPC; executable plugins are spawned as child processes
• Precedence: built-in > script plugin > executable plugin; project-level > user-level > PATH
• Security: handler file paths validated against path traversal; plugins that conflict with built-in command names are skipped with a warning
• Idempotent: with no plugins and no skill-extra.md, behavior is identical to before

Management commands

unityctl plugin list              # list all discovered plugins (script + executable)
unityctl plugin create <name>     # scaffold a script plugin (project-level)
unityctl plugin create <name> -g  # scaffold at user level (~/.unityctl/plugins/)
unityctl plugin remove <name>     # remove a script plugin
unityctl skill rebuild            # recompose SKILL.md with plugin docs

New files

File	Purpose
PluginManifest.cs	Plugin manifest model (plugin.json schema)
PluginLoader.cs	Discovers script plugins, creates System.CommandLine commands, generates skill sections
PluginCommands.cs	plugin list/create/remove management commands
ExecutablePluginLoader.cs	Discovers executable plugins, PATH fallback, process spawning with env vars
ContextHelper.cs	Shared helpers for extracting global options from InvocationContext
Resources/SKILL.md	Base skill (embedded resource, source of truth)
Resources/SKILL.plugins.md	Claude Code skill for plugin authoring guidance
.unityctl/plugins/sample-*	Sample plugins demonstrating both types

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 (script + executable)
• 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
• Plugin with name matching built-in command is skipped gracefully
• End-to-end: plugin command executes script in Unity via bridge
• End-to-end: executable plugin receives env vars and passes exit code through