fix(eval): include browser context in agent prompt (#530)

The eval's single-agent was passing raw task.query as the prompt,
without browser context (active tab URL, title). The agent didn't
know which page it was on, causing it to ask "which website?" instead
of browsing.

Use formatUserMessage() (same as chat-service.ts) to include browser
context in the prompt. Re-export formatUserMessage from agent/tool-loop.

packages/browseros-agent/apps/eval/src/agents/single-agent.ts

@@ -1,5 +1,8 @@
 import { randomUUID } from 'node:crypto'
-import { AiSdkAgent } from '@browseros/server/agent/tool-loop'
+import {
+  AiSdkAgent,
+  formatUserMessage,
+} from '@browseros/server/agent/tool-loop'
 import type { ResolvedAgentConfig } from '@browseros/server/agent/types'
 import { Browser } from '@browseros/server/browser'
 import { CdpBackend } from '@browseros/server/browser/backends/cdp'
@@ -91,8 +94,11 @@ export class SingleAgentEvaluator implements AgentEvaluator {
         capture,
         async (signal) => {
           if (!agent) throw new Error('Agent was not initialized')
+          // Format prompt with browser context so the agent knows what page it's on
+          // (same formatting as chat-service.ts → formatUserMessage)
+          const prompt = formatUserMessage(task.query, browserContext)
           const result = await agent.toolLoopAgent.generate({
-            prompt: task.query,
+            prompt,
             abortSignal: signal,
 
             experimental_onToolCallStart: ({ toolCall }) => {