Agent Stop: cancelling a conversation should stop server-side execution

Right now, stopping an agent conversation only cancels the frontend subscription. The agent keeps running server-side and all credits are consumed in full — even if the user stopped after 10 seconds of a 10-minute pipeline.

This makes the Stop button misleading. Users expect it to actually stop the execution, not just the UI.

Suggested fixes:

1. base44.agents.cancelConversation(conversationId) — SDK method to signal a hard stop server-side

2. A cancellation flag the agent checks between steps to exit early

3. At minimum — don't bill credits for tool executions that happen after a stop signal is received

This would make Stop actually mean Stop.