fix: Replace remaining agent.open() calls in voice and cache tests

Fixed agent.open() in:
- tests/magnitude/09-voice.mag.ts (4 instances)
- tests/magnitude/cache-success.mag.ts (1 instance)

All Magnitude tests now use the correct agent.act('Navigate to...') API.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

tests/magnitude/cache-success.mag.ts

@@ -8,7 +8,7 @@
 import { test } from 'magnitude-test';
 
 test('Node publishes successfully with cache (no warnings)', async (agent) => {
-  await agent.open('http://localhost:3000');
+  await agent.act('Navigate to http://localhost:3000');
 
   // Login
   await agent.act('Click the "Log in with Bluesky" button');