diagnostics: add event-level logging to foreground fallback

- Log every event that reaches handleEvent with type, sessionID, error preview
- Log tryFallback entry before any guards (with dedup/inProgress state)
- extractErrorPreview helper handles all event shapes (session.error,
  message.updated, session.status)

This lets us determine whether the proxy-level monthly usage limit
error reaches the plugin at all, and if so, its event type and shape.

src/hooks/foreground-fallback/index.ts

@@ -74,6 +74,46 @@ export function isRateLimitError(error: unknown): boolean {
 const DEDUP_WINDOW_MS = 5_000;
 const REPROMPT_DELAY_MS = 500;
 
+/**
+ * Extract a preview of the error field from event properties for diagnostic logging.
+ * Handles the different event shapes: session.error, message.updated, session.status.
+ */
+function extractErrorPreview(properties: unknown): string | undefined {
+  const p = properties as Record<string, unknown> | undefined;
+  if (!p) return undefined;
+
+  // session.error: { error: ApiError|... }
+  const err = p.error as Record<string, unknown> | undefined;
+  if (err) {
+    const msg =
+      (err.message as string) ??
+      ((err.data as Record<string, unknown> | undefined)?.message as
+        | string
+        | undefined);
+    return msg ?? `[${err.name ?? 'unknown error type'}]`;
+  }
+
+  // message.updated: { info: { error: ... } }
+  const info = p.info as Record<string, unknown> | undefined;
+  if (info?.error) {
+    const infoErr = info.error as Record<string, unknown>;
+    const msg =
+      (infoErr.message as string) ??
+      ((infoErr.data as Record<string, unknown> | undefined)?.message as
+        | string
+        | undefined);
+    return msg ?? `[${infoErr.name ?? 'unknown error type'}]`;
+  }
+
+  // session.status: { status: { message } }
+  const status = p.status as Record<string, unknown> | undefined;
+  if (status?.message) {
+    return status.message as string;
+  }
+
+  return undefined;
+}
+
 // ---------------------------------------------------------------------------
 // Manager
 // ---------------------------------------------------------------------------
@@ -116,6 +156,22 @@ export class ForegroundFallbackManager {
     const event = rawEvent as { type: string; properties?: unknown };
     if (!event?.type) return;
 
+    // Diagnostic: log every event reaching the fallback manager
+    {
+      const p = event.properties as Record<string, unknown> | undefined;
+      const sid =
+        (p?.sessionID as string | undefined) ??
+        ((p?.info as Record<string, unknown> | undefined)?.sessionID as
+          | string
+          | undefined);
+      const hasError = extractErrorPreview(event.properties);
+      log('[foreground-fallback] event', {
+        type: event.type,
+        sessionID: sid,
+        error: hasError,
+      });
+    }
+
     switch (event.type) {
       case 'message.updated': {
         const info = (
@@ -219,6 +275,11 @@ export class ForegroundFallbackManager {
   // ---------------------------------------------------------------------------
 
   private async tryFallback(sessionID: string): Promise<void> {
+    log('[foreground-fallback] tryFallback', {
+      sessionID,
+      inProgress: this.inProgress.has(sessionID),
+      dedupMs: Date.now() - (this.lastTrigger.get(sessionID) ?? 0),
+    });
     if (!sessionID) return;
     if (this.inProgress.has(sessionID)) return;