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/PluginCommands.kt

@@ -21,9 +21,24 @@ object PluginCommands : CompositeCommand(
         PluginConfig.reload()
         PluginData.reload()
         LargeLanguageModels.reload()
+        SkillStore.reload()
         sendMessage("OK")
     }
 
+    @SubCommand
+    suspend fun CommandSender.skills() {
+        val all = SkillStore.all
+        if (all.isEmpty()) {
+            sendMessage("暂无技能")
+            return
+        }
+        val response = buildString {
+            appendLine("当前技能（共 ${all.size} 个）：")
+            all.forEach { appendLine("- ${it.name}: ${it.description}") }
+        }
+        sendMessage(response.trim())
+    }
+
     @SubCommand
     suspend fun CommandSender.enable(contact: Contact) {
         when (contact) {