docs: surface Kijiji AND-matching behavior in tool, API, and MCP responses

Kijiji zero-result queries (e.g. 'macbook air m1 apple silicon') are
confusing because the failure mode is non-obvious. Surface the root
cause everywhere the caller can see it:
- MCP tool description warns about AND-matching and gives a concrete
  before/after example
- API 404 body includes the actionable hint via emptySearchResponse(hint)
- Core scraper logs the built URL and tip on page-1 zero results
- MCP handler unwraps the API message field so the hint reaches the LLM

packages/api-server/src/routes/kijiji.ts

@@ -73,7 +73,10 @@ export async function kijijiRoute(req: Request): Promise<Response> {
         { hideUnstableResults: true },
       );
       if (items.results.length === 0 && items.unstableResults.length === 0) {
-        return emptySearchResponse();
+        return emptySearchResponse(
+          `Kijiji matches ALL words in the query against listing titles. ` +
+            `Try a shorter or more common query (e.g. "macbook air m1" instead of "macbook air m1 apple silicon").`,
+        );
       }
       return Response.json(items, { status: 200 });
     }
@@ -86,7 +89,10 @@ export async function kijijiRoute(req: Request): Promise<Response> {
       {},
     );
     if (!items || items.length === 0) {
-      return emptySearchResponse();
+      return emptySearchResponse(
+        `Kijiji matches ALL words in the query against listing titles. ` +
+          `Try a shorter or more common query (e.g. "macbook air m1" instead of "macbook air m1 apple silicon").`,
+      );
     }
     return Response.json(items, { status: 200 });
   } catch (error) {