fix(replay): preserve agent on fallback prompt replay

Pass `agent` in the session.prompt body so the fallback model runs
with the same agent as the original request. Without this, OpenCode
defaults to its built-in plan agent (read-only mode), causing the
fallback model to refuse tool execution.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: blazejpawlak

src/replay/orchestrator.ts

@@ -214,6 +214,7 @@ export async function attemptFallback(
         path: { id: sessionId },
         body: {
           model: { providerID, modelID },
+          agent: agentName ?? undefined,
           parts: promptParts as NonNullable<
             Parameters<typeof client.session.prompt>[0]["body"]
           >["parts"],