W20: Tool Calling 闭环 + Stream+Tools + 回归测试 + session auto-save + ASan CI (W20.1-W20.6)

- W20.1: CLI tool_calls→execute→result→re-call 循环（5轮上限）
- W20.2: deepseek 流式 tool_calls 增量解析（configure 缓存，无 ABI break）
- W20.3: plugin_loader 回归测试 5 块 32 断言（路径/原子性/mock 日志）
- W20.4: plugin_loader ABI 契约校验（name/version/on_init 字段验证）
- W20.5: ASan/UBSan CMake preset + CI sanitizer job（PR-only Linux）
- W20.6: session auto-save（on_shutdown 写 %APPDATA%/dstalk/session.json）

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

plugins/session/src/session_plugin.cpp

@@ -12,6 +12,7 @@
 #include <atomic>
 #include <cstddef>
 #include <cstdint>
+#include <cstdlib>
 #include <cstring>
 #include <exception>
 #include <mutex>
@@ -287,6 +288,24 @@ static dstalk_session_service_t g_session_service = {
     session_token_count
 };
 
+// ============================================================
+// W20.6: 默认会话保存路径（平台标准目录）
+// ============================================================
+
+static std::string get_default_session_path() {
+#ifdef _WIN32
+    char* buf = nullptr;
+    size_t len = 0;
+    _dupenv_s(&buf, &len, "APPDATA");
+    std::string dir = buf ? std::string(buf) + "/dstalk" : "dstalk";
+    free(buf);
+#else
+    const char* home = std::getenv("HOME");
+    std::string dir = home ? std::string(home) + "/.dstalk" : "/tmp/dstalk";
+#endif
+    return dir + "/session.json";
+}
+
 // ============================================================
 // 插件生命周期
 // ============================================================
@@ -304,7 +323,13 @@ static int on_init(const dstalk_host_api_t* host) {
         g_file_io.store(static_cast<const dstalk_file_io_service_t*>(raw), std::memory_order_release);
 
         // 注册自身服务
-        return host->register_service("session", 1, &g_session_service);
+        int ret = host->register_service("session", 1, &g_session_service);
+        if (ret != 0) return ret;
+
+        // W20.6: 从默认路径恢复会话（文件不存在则静默失败）
+        session_load(get_default_session_path().c_str());
+
+        return 0;
     } catch (const std::exception& e) {
         const dstalk_host_api_t* h = g_host.load(std::memory_order_acquire);
         if (h) h->log(DSTALK_LOG_ERROR, "on_init[session]: %s", e.what());
@@ -318,6 +343,9 @@ static int on_init(const dstalk_host_api_t* host) {
 
 static void on_shutdown() {
     try {
+        // W20.6: 清空前自动保存到默认路径
+        session_save(get_default_session_path().c_str());
+
         std::lock_guard<std::mutex> lock(g_session_mutex);
         rebuild_cached_history_locked();
         g_cached_history.clear();