docs: add agent skill guides and update CMake build instructions

.agents/skills/testing_guide.md

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+# Skill: Standardized Testing & Validation
+
+This skill provides the standard workflow for testing and validating changes in the `uLib` project.
+
+## Context
+- **Tooling**: `ctest` and direct execution of test binaries in the `build/` directory.
+- **Location**: Test binaries are typically located in `build/src/*/testing/` or `build/Testing/`.
+
+## Workflow
+
+### 1. Running All Tests
+From the root directory:
+```bash
+ctest --test-dir build/clang-make --output-on-failure
+```
+
+### 2. Running Component Tests
+Run specific categories of tests:
+- **Core**: `./build/clang-make/src/Core/testing/CoreTest`
+- **Math**: `./build/clang-make/src/Math/testing/MathVectorTest`
+- **Geant**: `./build/clang-make/src/HEP/Geant/testing/GeantApp`
+- **VTK**: `./build/clang-make/src/Vtk/testing/vtkViewerTest`
+
+### 3. Debugging a Failing Test
+Run the binary directly through `gdb` or `valgrind` (if available):
+```bash
+gdb --args ./build/clang-make/src/Core/testing/ObjectWrapperTest
+```
+
+## Validation Checklist for New Features
+- [ ] Does `ctest` pass globally?
+- [ ] If changing visualization, does `vtkViewerTest` show the correct results?
+- [ ] If changing Geant logic, does `GeantApp` run without memory aborts?
+- [ ] Are new tests added to the appropriate `CMakeLists.txt`?