Add global skill system for self-accumulated knowledge

Introduce a cross-group skill system that lets the bot distill reusable
knowledge into markdown docs and load them on demand, keeping day-to-day
context pollution low.

- SkillStore manages data/skills/*.md files with name/description
  frontmatter and an in-memory index cache (rebuilt on init/reload)
- Only the skill index (name + one-line description) is injected via the
  new {skills} system-prompt placeholder; bodies load on demand
- New tools: loadSkill / saveSkill (upsert, iterate = load+overwrite) /
  deleteSkill, gated by PluginConfig.skillsEnabled
- Skill names validated against ^[A-Za-z0-9_-]+$ to prevent traversal
- Wire SkillStore.init into onEnable and refresh on /jgpt reload; add
  /jgpt skills listing command
- Bump version to 1.12.0; update README

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/main/kotlin/PluginConfig.kt

@@ -123,6 +123,9 @@ object PluginConfig : AutoSavePluginConfig("Config") {
     @ValueDescription("是否启用记忆编辑功能，记忆存在data目录，提示词中需要加上{memory}来填充记忆，每个群都有独立记忆")
     val memoryEnabled by value(true)
 
+    @ValueDescription("是否启用技能系统，技能存在data/skills目录（全局跨群），提示词中需要加上{skills}来注入技能索引")
+    val skillsEnabled by value(true)
+
     @ValueDescription("是否启用好感度系统")
     val enableFavorabilitySystem by value(true)