W22: coverage metric + network tests + Tool stream feedback + stdin pipe + session path + dependency check (W22.1-W22.6)

- W22.1: gcovr 覆盖率度量 + CI coverage job（40% 阈值 warning）
- W22.2: network_plugin 单元测试（parse_headers_json/extract_host_port/SSE/异常保护）
- W22.3: Tool Calling 流式反馈（chat_stream + "[工具调用]/[工具结果]" 状态行）
- W22.4: --prompt stdin pipe（--prompt - 从 stdin 读取）
- W22.5: session 路径健壮化（static 缓存 + mkdir + fallback）
- W22.6: 插件依赖拓扑静态校验（validate_dependencies 循环/缺失检测）

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

plugins/session/src/session_plugin.cpp

@@ -15,6 +15,7 @@
 #include <cstdlib>
 #include <cstring>
 #include <exception>
+#include <filesystem>
 #include <mutex>
 #include <string>
 #include <utility>
@@ -293,17 +294,32 @@ static dstalk_session_service_t g_session_service = {
 // ============================================================
 
 static std::string get_default_session_path() {
+    // W22.5: static 缓存 + mkdir 保障 + 失败 fallback 到当前目录
+    static std::string cached_path = []() -> std::string {
 #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);
+        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";
+        const char* home = std::getenv("HOME");
+        std::string dir = home ? std::string(home) + "/.dstalk" : "/tmp/dstalk";
 #endif
-    return dir + "/session.json";
+
+        std::error_code ec;
+        std::filesystem::create_directories(dir, ec);
+        if (ec) {
+            const dstalk_host_api_t* host = g_host.load(std::memory_order_acquire);
+            if (host) host->log(DSTALK_LOG_WARN,
+                "get_default_session_path: cannot mkdir '%s' (%s), fallback to .",
+                dir.c_str(), ec.message().c_str());
+            return std::string("./session.json");
+        }
+
+        return dir + "/session.json";
+    }();
+    return cached_path;
 }
 
 // ============================================================