fix(v2): default systemPrompt to claude_code preset for CLAUDE.md loading

Without the preset, settingSources loads files but the system prompt has
no slot to inject CLAUDE.md content. The claude_code preset enables the
full Claude Code system prompt including project instructions.

v2/sidecar/agent-runner-deno.ts

@@ -103,7 +103,9 @@ async function handleQuery(msg: QueryMessage) {
         permissionMode: (permissionMode ?? "bypassPermissions") as "bypassPermissions" | "default",
         allowDangerouslySkipPermissions: (permissionMode ?? "bypassPermissions") === "bypassPermissions",
         settingSources: settingSources ?? ["user", "project"],
-        systemPrompt: systemPrompt ?? undefined,
+        systemPrompt: systemPrompt
+          ? systemPrompt
+          : { type: "preset" as const, preset: "claude_code" as const },
         model: model ?? undefined,
         additionalDirectories: additionalDirectories ?? undefined,
       },