Previously, plugin skills were only available via slash commands (/plugin:skill)
but not through the skill tool (mcp_skill). This was because:
- loadPluginSkillsAsCommands() returned Record<string, CommandDefinition>
- discoverSkills() did not include plugin skills
- mergeSkills() never received plugin skills

This change:
- Adds discoverPluginSkills() that returns LoadedSkill[]
- Integrates plugin skills into discoverSkills() and discoverAllSkills()
- Changes PluginComponentsResult.skills type to LoadedSkill[]
- Adds 'plugin' to SkillScope with appropriate priority
- Maintains backward compatibility for slash commands

Now plugin skills are accessible through both:
- Slash commands: /plugin:skill-name
- Skill tool: mcp_skill with automatic discovery