Step 247: file operation tools for MCP (read, write, create, diff, list)

Adds 5 workspace-aware file I/O tools to the MCP server so agents can
manage files without a GUI. All paths are resolved against --workspace
root with escape-rejection security. 12/12 tests pass, tool count now 15.

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

editor/src/HeadlessEditorState.h

@@ -22,6 +22,7 @@
 #include "AgentCodeGen.h"
 #include "AgentLibraryPolicy.h"
 #include "WorkflowRecorder.h"
+#include "FileOperations.h"
 #include "Orchestrator.h"
 #include "IncrementalOptimizer.h"
 #include "TextASTSync.h"