fix: correct V2 SDK API to use receive() instead of stream()

[Copilot]

The V2 Agent SDK implementation was calling sdkSession.stream() instead of the documented sdkSession.receive() method, causing a mismatch with the official API specification.

Changes

• SDKAgent.ts (lines 129, 176): Changed stream() → receive() for AsyncGenerator message iteration
• Comments: Updated to reflect correct V2 "receive" terminology

API Alignment

The Session interface per official V2 docs:

interface Session {
  send(message: string): Promise<void>;
  receive(): AsyncGenerator<SDKMessage>;  // Not stream()
  close(): void;
}

Before:

await sdkSession.send(initPrompt);
for await (const message of sdkSession.stream()) {  // ❌ Incorrect
  await this.handleSDKMessage(message, session, worker);
}

After:

await sdkSession.send(initPrompt);
for await (const message of sdkSession.receive()) {  // ✅ Correct
  await this.handleSDKMessage(message, session, worker);
}

All other V2 API usage (unstable_v2_createSession, unstable_v2_resumeSession, unstable_v2_prompt, await using cleanup) was already correct.

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