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/JChatGPT.kt

@@ -52,7 +52,7 @@ object JChatGPT : KotlinPlugin(
     JvmPluginDescription(
         id = "top.jie65535.mirai.JChatGPT",
         name = "J ChatGPT",
-        version = "1.11.0",
+        version = "1.12.0",
     ) {
         author("jie65535")
 //        dependsOn("xyz.cssxsh.mirai.plugin.mirai-hibernate-plugin", true)
@@ -82,6 +82,9 @@ object JChatGPT : KotlinPlugin(
         // 初始化 token 使用日聚合存储（独立 JSON 文件，绕开 yamlkt 大数据 bug）
         TokenUsageStore.init(dataFolder)
 
+        // 初始化技能存储（data/skills/ 下的 markdown 文件，全局跨群）
+        SkillStore.init(dataFolder)
+
         // 设置Token
         LargeLanguageModels.reload()
 
@@ -269,6 +272,12 @@ object JChatGPT : KotlinPlugin(
             } else memoryText
         }
 
+        replace("{skills}") {
+            if (PluginConfig.skillsEnabled) {
+                SkillStore.buildIndexPrompt()
+            } else "暂无技能"
+        }
+
         replace("{meme}") {
             memePrompt?.let { return@replace it }
 
@@ -928,6 +937,15 @@ object JChatGPT : KotlinPlugin(
         // 记忆修改
         MemoryReplace(),
 
+        // 技能：加载
+        LoadSkill(),
+
+        // 技能：沉淀/迭代
+        SaveSkill(),
+
+        // 技能：删除
+        DeleteSkill(),
+
         // 搜索聊天历史
         SearchChatHistory(),