chore: remove plugin from repo, add plugin guide, docs + AGPLv3 license

- mc-report-plugin removed from git tracking (kept locally), gitignored;
  plugin code no longer distributed (old x-external-key auth, outdated)
- docs/PLUGIN-GUIDE.md: Java plugin integration guide using new
  ID+Secret -> SESSION auth (session mgmt, submit ticket, track, bans)
- README: updated external API table (SESSION), dynamic sources,
  removed obsolete plugin commands/tech row, new dir structure, doc links
- INSTALL: drop restart requirement, add upgrade-migration/log/sources
  sections, license section
- LICENSE: GNU AGPL v3 full text
- verified: 24 checks (git tracking, docs consistency, license integrity)

.gitignore

@@ -1,10 +1,10 @@
 node_modules/
-# 本地运行配置与上传文件(含数据库密码,严禁入库)
 data/
-# 日志与临时文件
 *.log
 *.tmp.js
 *.tmp.sh
 *.bak
 # 会话记录
 session-*.md
+# MC 插件代码不随仓库分发(见 docs/PLUGIN-GUIDE.md 示例自行实现)
+mc-report-plugin/