feat(core): add getErrorMessage helper for narrowing unknown errors

Designed to replace the \`catch (err: any) { ... err.message ... }\` pattern
across the repo. Keeps the catch binding typed as \`unknown\` (TS default)
while still producing a readable message.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/core/src/error-message.ts

@@ -0,0 +1,17 @@
+/**
+ * Narrow an unknown caught error into a string message.
+ *
+ * Designed to replace the `catch (err: any) { ... err.message ... }` pattern:
+ * prefer `catch (err) { toast(getErrorMessage(err)) }` — keeps the binding
+ * typed as `unknown` (TS default with useUnknownInCatchVariables) while
+ * still producing a readable message.
+ */
+export function getErrorMessage(err: unknown): string {
+  if (err instanceof Error) return err.message;
+  if (typeof err === "string") return err;
+  try {
+    return JSON.stringify(err);
+  } catch {
+    return String(err);
+  }
+}

packages/core/src/index.ts

@@ -596,3 +596,6 @@ export type {
 } from "./chat-types.js";
 export { ChatStore } from "./chat-store.js";
 export type { ChatStoreEvents } from "./chat-store.js";
+
+// ── Error helpers ─────────────────────────────────────────
+export { getErrorMessage } from "./error-message.js";