build: modernize repo-lens skeleton and cross-platform configuration

Replace CPM with FetchContent and update GoogleTest to v1.18.0.
Add compiler toolchains and Debug, Release, and sanitizer presets.
Require CMake 4.0, use target properties for compiler settings,
and enable compilation database export.

Fix skeleton compilation errors and adopt trailing return types,
nodiscard execute methods, and noexcept signatures. Update CI
and project documentation without implementing any TODOs.

Validated Windows builds and tests in all three configurations.

include/repo_tool_interface.hpp

@@ -0,0 +1,29 @@
+#ifndef REPO_LENSE_TOOL_INTERFACE_HPP_INCLUDED
+#define REPO_LENSE_TOOL_INTERFACE_HPP_INCLUDED
+
+#include <cstdint>
+#include <string_view>
+
+#pragma once
+
+namespace rl
+{
+
+using exit_code = std::int32_t;
+
+class IRepoLensTool
+{
+public: 
+    IRepoLensTool() noexcept = default;
+    virtual ~IRepoLensTool() noexcept = default;
+    IRepoLensTool(const IRepoLensTool&) noexcept = delete;
+    IRepoLensTool(IRepoLensTool&&) noexcept = delete;
+    auto operator=(const IRepoLensTool&) noexcept -> IRepoLensTool& = delete;
+    auto operator=(IRepoLensTool&&) noexcept -> IRepoLensTool& = delete;
+
+    [[nodiscard]] virtual auto execute(std::string_view arguments) noexcept -> exit_code = 0;
+};
+
+}
+
+#endif //REPO_LENSE_TOOL_INTERFACE_HPP_INCLUDED