refactor(core): extract core modules (state, persistence, config, events)

Extract core system modules from monolithic index.js into modular architecture:

- src/core/state.js: All extension state variables with controlled setters
- src/core/persistence.js: Settings and chat data persistence functions
- src/core/config.js: Extension metadata and default configuration
- src/core/events.js: SillyTavern event system wrapper

Updated index.js to import and use new core modules.
Removed ~220 lines of state/persistence code from index.js.

Part of Epic 1: Foundation & Core Systems (Phase 1.1-1.2)

src/core/events.js

@@ -0,0 +1,47 @@
+/**
+ * Core Events Module
+ * Wrapper for SillyTavern event system
+ */
+
+import { eventSource, event_types } from '../../../../../../script.js';
+
+/**
+ * Register an event handler
+ * @param {string} eventType - Event type from event_types
+ * @param {Function} handler - Event handler function
+ */
+export function on(eventType, handler) {
+    eventSource.on(eventType, handler);
+}
+
+/**
+ * Register a one-time event handler
+ * @param {string} eventType - Event type from event_types
+ * @param {Function} handler - Event handler function
+ */
+export function once(eventType, handler) {
+    eventSource.once(eventType, handler);
+}
+
+/**
+ * Remove an event handler
+ * @param {string} eventType - Event type from event_types
+ * @param {Function} handler - Event handler function to remove
+ */
+export function off(eventType, handler) {
+    eventSource.off(eventType, handler);
+}
+
+/**
+ * Emit an event
+ * @param {string} eventType - Event type to emit
+ * @param {...*} args - Arguments to pass to handlers
+ */
+export function emit(eventType, ...args) {
+    eventSource.emit(eventType, ...args);
+}
+
+/**
+ * Re-export event types for convenience
+ */
+export { event_types };