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>
