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 <noreply@anthropic.com>

src/main/kotlin/tools/SaveSkill.kt

@@ -0,0 +1,62 @@
+package top.jie65535.mirai.tools
+
+import com.aallam.openai.api.chat.Tool
+import com.aallam.openai.api.core.Parameters
+import kotlinx.serialization.json.JsonObject
+import kotlinx.serialization.json.add
+import kotlinx.serialization.json.jsonPrimitive
+import kotlinx.serialization.json.put
+import kotlinx.serialization.json.putJsonArray
+import kotlinx.serialization.json.putJsonObject
+import net.mamoe.mirai.event.events.MessageEvent
+import top.jie65535.mirai.JChatGPT
+import top.jie65535.mirai.PluginConfig
+import top.jie65535.mirai.SkillStore
+
+/**
+ * 新增或整篇覆盖一个技能（全局，跨群共享）。
+ * 用于把群里学到/被纠正的知识沉淀下来；迭代时先用 loadSkill 读全文，改好后整篇写回。
+ */
+class SaveSkill : BaseAgent(
+    tool = Tool.function(
+        name = "saveSkill",
+        description = "沉淀或更新一个技能（知识文档），全局跨群共享。新增直接写；迭代时先 loadSkill 读全文，修改后整篇写回。技能名相同则覆盖。",
+        parameters = Parameters.buildJsonObject {
+            put("type", "object")
+            putJsonObject("properties") {
+                putJsonObject("name") {
+                    put("type", "string")
+                    put("description", "技能名，kebab-case，只能含字母/数字/下划线/连字符，如 kubejs-basics。相同则覆盖")
+                }
+                putJsonObject("description") {
+                    put("type", "string")
+                    put("description", "一句话简介，会常驻技能索引，决定你以后何时加载它")
+                }
+                putJsonObject("content") {
+                    put("type", "string")
+                    put("description", "技能正文（markdown），沉淀的知识、经验或提示词。整篇内容，会覆盖旧版本")
+                }
+            }
+            putJsonArray("required") {
+                add("name")
+                add("description")
+                add("content")
+            }
+        }
+    )
+) {
+    override val isEnabled: Boolean
+        get() = PluginConfig.skillsEnabled
+
+    override val loadingMessage: String = "记下来了..."
+
+    override suspend fun execute(args: JsonObject?, event: MessageEvent): String {
+        requireNotNull(args)
+        val name = args.getValue("name").jsonPrimitive.content
+        val description = args.getValue("description").jsonPrimitive.content
+        val content = args.getValue("content").jsonPrimitive.content
+        JChatGPT.logger.info("Save skill: \"$name\" - \"$description\"")
+        val error = SkillStore.save(name, description, content)
+        return error ?: "OK，技能 \"$name\" 已保存。"
+    }
+}