feat: add support code completion via acp

sources/completion/CompletionEngine.hpp

@@ -0,0 +1,34 @@
+// Copyright (C) 2026 Petr Mironychev
+// SPDX-License-Identifier: GPL-3.0-or-later
+// Additional attribution terms under GPLv3 §7(b) apply — see LICENSE
+
+#pragma once
+
+#include <QObject>
+#include <QString>
+
+namespace QodeAssist {
+
+struct CompletionContext
+{
+    QString filePath;
+    int line = 0;
+    int column = 0;
+};
+
+class CompletionEngine : public QObject
+{
+    Q_OBJECT
+
+public:
+    using QObject::QObject;
+
+    virtual void request(quint64 requestId, const CompletionContext &context) = 0;
+    virtual void cancel(quint64 requestId) = 0;
+
+signals:
+    void completionReady(quint64 requestId, const QString &text);
+    void completionFailed(quint64 requestId, const QString &error);
+};
+
+} // namespace QodeAssist