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/mcp-server/src/protocol/tools.ts

@@ -11,7 +11,11 @@ export const tools = [
       properties: {
         query: {
           type: "string",
-          description: "Search query for Kijiji listings",
+          description:
+            "Search query for Kijiji listings. " +
+            "Kijiji requires ALL words to appear in the listing title — keep queries short and use terms sellers actually write. " +
+            "Avoid marketing/brand phrases sellers don't use (e.g. use 'macbook air m1' not 'macbook air m1 apple silicon'). " +
+            "If the search returns no results, try a shorter or more common query.",
         },
         location: {
           type: "string",