Complete build wiring and CLI file commands

Align documented commands with the CLI, enable optional GUI/test targets, and remove committed API secrets so the project is safer to build and run.

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

tests/smoke_test.cpp

@@ -0,0 +1,52 @@
+#include "dstalk/dstalk_api.h"
+
+#include <cstring>
+#include <filesystem>
+#include <fstream>
+#include <iostream>
+#include <string>
+
+int main()
+{
+    const auto dir = std::filesystem::temp_directory_path() / "dstalk-smoke-test";
+    std::filesystem::create_directories(dir);
+
+    const auto config_path = dir / "config.toml";
+    {
+        std::ofstream config(config_path);
+        config << "[api]\n"
+               << "provider = \"deepseek\"\n"
+               << "base_url = \"https://api.deepseek.com/v1\"\n"
+               << "api_key = \"test-key\"\n"
+               << "model = \"deepseek-chat\"\n";
+    }
+
+    if (dstalk_init(config_path.string().c_str()) != 0) {
+        std::cerr << "dstalk_init failed\n";
+        return 1;
+    }
+
+    const auto file_path = dir / "sample.txt";
+    if (dstalk_file_write(file_path.string().c_str(), "hello dstalk") != 0) {
+        std::cerr << "dstalk_file_write failed\n";
+        dstalk_destroy();
+        return 1;
+    }
+
+    char* content = nullptr;
+    if (dstalk_file_read(file_path.string().c_str(), &content) != 0 || !content) {
+        std::cerr << "dstalk_file_read failed\n";
+        dstalk_destroy();
+        return 1;
+    }
+
+    const bool ok = std::strcmp(content, "hello dstalk") == 0;
+    dstalk_free_string(content);
+    dstalk_destroy();
+
+    if (!ok) {
+        std::cerr << "unexpected file content\n";
+        return 1;
+    }
+    return 0;
+}