feat(integration): extract SillyTavern event handlers to dedicated module

- Create src/systems/integration/sillytavern.js with all event handlers
- Move commitTrackerData() (deferred from Epic 1)
- Move sendPlotProgression() to plotProgression.js
- Move updateGenerationModeUI() to layout.js
- Add registerAllEvents() and unregisterAllEvents() to events.js
- Centralize event registration in index.js initialization

This completes Epic 6: Integration Layer Extraction
~340 lines extracted from index.js
index.js reduced from ~783 lines to 423 lines

src/core/events.js

@@ -45,3 +45,44 @@ export function emit(eventType, ...args) {
  * Re-export event types for convenience
  */
 export { event_types };
+
+// Store registered handlers for cleanup
+const registeredHandlers = new Map();
+
+/**
+ * Registers all extension event handlers
+ * @param {Object} handlers - Map of event types to handler functions or arrays of handler functions
+ * @example
+ * registerAllEvents({
+ *     [event_types.MESSAGE_SENT]: onMessageSent,
+ *     [event_types.CHAT_CHANGED]: [onCharacterChanged, updatePersonaAvatar]
+ * });
+ */
+export function registerAllEvents(handlers) {
+    for (const [eventType, handler] of Object.entries(handlers)) {
+        // Handler can be a single function or an array of functions
+        const handlerArray = Array.isArray(handler) ? handler : [handler];
+
+        for (const handlerFn of handlerArray) {
+            eventSource.on(eventType, handlerFn);
+
+            // Store for later cleanup
+            if (!registeredHandlers.has(eventType)) {
+                registeredHandlers.set(eventType, []);
+            }
+            registeredHandlers.get(eventType).push(handlerFn);
+        }
+    }
+}
+
+/**
+ * Unregisters all extension event handlers (for cleanup/reload)
+ */
+export function unregisterAllEvents() {
+    for (const [eventType, handlers] of registeredHandlers.entries()) {
+        for (const handler of handlers) {
+            eventSource.off(eventType, handler);
+        }
+    }
+    registeredHandlers.clear();
+}