fix: add core web vitals to performance_start_trace description

Fixes #124 and #159. People have reported that when asked for CWV
metrics, the AI will use `evaluate_script` to inject some code into the
page to gather them. But, we want to encourage it to record a trace
instead and use those values.

After some testing locally, adding CWV explicitly to the description
makes it much more reliable that the AI will reach for trace recording
when asked about CWVs.

Author: jackfranklin

src/tools/performance.ts

@@ -22,7 +22,8 @@ import {defineTool} from './ToolDefinition.js';
 
 export const startTrace = defineTool({
   name: 'performance_start_trace',
-  description: 'Starts a performance trace recording on the selected page.',
+  description:
+    'Starts a performance trace recording on the selected page. This can be used to look for performance problems and insights to improve the performance of the page. It will also report Core Web Vital (CWV) scores for the page.',
   annotations: {
     category: ToolCategories.PERFORMANCE,
     readOnlyHint: true,
@@ -31,7 +32,7 @@ export const startTrace = defineTool({
     reload: z
       .boolean()
       .describe(
-        'Determines if, once tracing has started, the page should be automatically reloaded',
+        'Determines if, once tracing has started, the page should be automatically reloaded.',
       ),
     autoStop: z
       .boolean()