fix: move AbortSignal from API request body to SDK options

[galligan]

Summary

AgentService.runAgentLoop passes signal inside the Anthropic API request body object. The API rejects this with a 400:

signal: Extra inputs are not permitted

signal is a client-side SDK option for request cancellation, not an API parameter. It belongs in the second argument to client.messages.create().

Before:

const request = { model, messages, tools, signal: this.abortController?.signal };
result = await this.client!.messages.create(request);

After:

const request = { model, messages, tools };
result = await this.client!.messages.create(request, {
  signal: this.abortController?.signal ?? undefined,
});

One-line fix. The abort signal still works for cancellation — it's just passed in the right place now.

Test plan

• "Ask Proof" agent dialog no longer fails with "Extra inputs are not permitted"
• Cancelling an in-progress agent request still aborts the API call

Closes #26

🤘🏻 In-collaboration-with: Claude Code