docs: add agent skill guides and update CMake build instructions

.agents/skills/geant_simulation.md

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+# Skill: HEP/Geant Simulation Rules
+
+This skill provides instructions for developing the Geant4 simulation components within `uLib`.
+
+## Context
+- **Domain Objects**: `Material`, `Solid`, `LogicalVolume`, `PhysicalVolume`.
+- **Integration**: `mutomGeant` library wraps Geant4 classes into `uLib::Object`s.
+
+## Patterns
+
+### 1. Adding a New Solid
+New solids must implement `GetPolyhedron()` to support VTK visualization.
+```cpp
+G4Polyhedron* MySolid::GetPolyhedron() const {
+    // Return the tessellated representation of the Geant4 solid
+    return m_G4Solid->GetPolyhedron();
+}
+```
+
+### 2. Physical Volume Hierarchy
+Maintain the relationship between `PhysicalVolume` and its parent `LogicalVolume`.
+```cpp
+auto* world = new LogicalVolume(worldSolid, worldMat);
+auto* detector = new PhysicalVolume(detectorLogic, world, "Detector1");
+detector->SetPosition({0, 0, 100}); // Relative to parent
+```
+
+### 3. Transformation Synchronization
+Use the centralized `TRS` object to manage position and rotation. Synchronization with Geant4's internal stores should be reactive.
+- Listen to `Object::Updated` on the `Solid` or `PhysicalVolume`.
+- Update the underlying `G4VPhysicalVolume` position/rotation.
+
+## Material Management
+Use the `Matter` class to manage Geant4 materials. Ensure materials are registered in the `G4NistManager` or custom material store if needed.
+
+## Checklist
+- [ ] Does the solid implement `GetPolyhedron()`?
+- [ ] Are parents correctly assigned in `PhysicalVolume` constructors?
+- [ ] Is the `TRS` object used for all spatial transformations?