Step 245: HeadlessEditorState — agent API surface without ImGui/SDL

Adds the headless agent architecture for Sprint 9 Phase 9a:
- ASTUtils.h: pure AST utilities extracted from EditorUtils.h
- HeadlessEditorState.h: GUI-free state with buffer management
- HeadlessAgentRPCHandler.h: full RPC dispatch (20+ methods)
- step245_test.cpp: 20 tests all passing

Also fixes test compilation errors (NotificationSystem API changes,
AgentRole permissions, DependencyPanel missing include) and adds
SDL2 system library detection fix in CMakeLists.txt.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

editor/src/DependencyPanel.h

@@ -8,6 +8,7 @@
 #include <fstream>
 #include <cstdio>
 #include <sstream>
+#include <unordered_set>
 #include <vector>
 #include <string>
 #include <unordered_map>