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]>

build.gradle.kts

@@ -7,7 +7,7 @@ plugins {
 }
 
 group = "top.jie65535.mirai"
-version = "1.11.0"
+version = "1.12.0"
 
 mirai {
     jvmTarget = JavaVersion.VERSION_11