W18: context cleanup + CLI fixes + loader audit + CI matrix (W18.1-W18.4)

- W18.1 (王测+林深): Remove g_max_tokens dead API, UTF-8 bounds protection, deduplicate token counting, 0xC0/0xC1 handling, add 13 test blocks (36 checks)
- W18.2 (赵码+朱晴): Fix /context no-session error message, /status 3-state connection display
- W18.3 (曹武+徐磊): plugin_loader security audit — 9 dimensions, rating C, 1 HIGH + 2 MEDIUM findings
- W18.4 (马奔+胡桐): CI dual-platform matrix (Ubuntu clang-18 + Windows clang-cl), ccache, build timing baseline

Build 0 error, ctest 5/5 pass, metadata check clean.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

agents/architect-lin/profile.md

@@ -42,6 +42,9 @@ performance_log:
   - date: 2026-05-27
     event: "W14.4 完成：诊断 W12.2 双 store 整合未生效根因——测试加载了 build/tests/plugins/ 下 pre-W12.2 的旧 DLL，而非 build/plugins/ 下 post-W12.2 新 DLL。修复：host.cpp 新增插件目录 fallback 搜索（plugins/ -> ../plugins/），清理 build/tests/ 下陈旧产物。build 0 error，4/4 test pass，R2 由 WARN 变 PASS"
     rating: completed
+  - date: 2026-05-27
+    event: "W18.1 (协作 王测): 关闭 F-11.1-3/4/5/6 共4条 context_plugin 遗留发现。(3) 删除 g_max_tokens + context_set_max_tokens API，更新 dstalk_services.h 移除 vtable 字段，trim_impl 硬编码默认值 4096 + max_msg_count 改用 max_tokens 参数；(4) count_tokens_utf8 共享函数新增多字节序列越界保护（i+N>=len + 后继字节 & 0xC0 校验）；(5) 提取 count_tokens_utf8 消除 count_tokens_one_message/count_tokens_trim 双份重复；(6) c==0xC0||0xC1 独立分支检测过短编码。新增 context_plugin_test.cpp 13 测试块。编译 0 error + ctest 5/5 pass。"
+    rating: completed
 current_groups:
   - grp-quality-core (成员)
   - grp-ai-plugins (待命)