docs: add agent skill guides and update CMake build instructions

.agents/skills/signal_bridge.md

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+# Skill: Multi-System Signaling (uLib ↔ Qt)
+
+This skill manages the coexistence of `uLib::Object` signals and Qt's `Q_OBJECT` signaling system.
+
+## Context
+- **uLib Signals**: Used for domain logic and data changes (`uLib::Object::connect`).
+- **Qt Signals**: Used for UI events, widgets, and application-level control flow (`QObject::connect`).
+
+## Patterns
+
+### 1. Bridging Logic
+When a domain change needs to trigger a UI update, use a wrapper or a direct connection if the widget has access to the `uLib::Object`.
+```cpp
+// In a Qt Widget
+uLib::Object::connect(domainObj, &Object::Updated, [this]() {
+    this->update(); // Trigger Qt repaint
+});
+```
+
+### 2. Selection Flow
+Selection usually starts in the VTK Viewport (Qt) and flows to the domain context.
+1. `QViewport` emits `prop3dSelected(Prop3D*)` (Qt signal).
+2. `MainPanel` catches it and calls `contextPanel->selectObject(p->GetContent())`.
+3. `ContextPanel` updates the tree view and property editors.
+
+### 3. Connection Hygiene
+- Use `uLib::Object::connect` for everything involving `uLib::Property` changes.
+- Use Qt `connect` for button clicks, menu actions, and window events.
+- Be careful with lambda captures; ensure the captured object is still alive or use weak pointers if necessary.
+
+## Checklist
+- [ ] Is the correct signaling system being used for the task?
+- [ ] Are capture groups in lambdas safe?
+- [ ] Does selection flow correctly between the 3D view and the tree view?