[FEATURE]: Evaluate subagent as an opt-in Code Mode plugin tool

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Describe the enhancement you want to request

Evaluate subagent as an opt-in Code Mode tool through the plugin API.

This is a narrow follow-up to #43137, where built-in Code Mode exposure was intentionally left plugin-first until there are more evals. It is not a request to enable every built-in tool or add a second workflow scheduler.

The experiment should adapt the existing SubagentTool execution path so Code Mode can compose independent child calls, including Promise.all, while preserving its existing permissions, parent/child session links, depth limits, cancellation, and logging.

A useful first contract would be:

const results = await Promise.all([
  tools.subagent({ agent: "explore", description: "trace auth flow", prompt: "..." }),
  tools.subagent({ agent: "explore", description: "inspect tests", prompt: "..." }),
])

return results.map(({ sessionID, status, summary }) => ({
  sessionID,
  status,
  summary,
}))

Acceptance/evaluation cases:

• independent calls actually overlap, subject to the existing Code Mode concurrency cap;
• each result includes a durable child session ID and explicit terminal status;
• one child failure is visible and does not erase successful siblings;
• aborting the parent cancels in-flight children through the existing lifecycle;
• child output is returned as a bounded structured result rather than dumping full transcripts into parent context;
• nested subagent delegation remains blocked by the existing depth/permission policy;
• direct subagent behavior is unchanged when the plugin is disabled.

This follows the current architecture: Code Mode composes typed tool calls; the existing session/job runtime remains responsible for agent lifecycle. It should also help determine which public session/job capabilities in #34957 are actually required before considering any default native exposure.

Context: https://x.com/thdxr/status/2094828159308878216 and https://x.com/thdxr/status/2094831285986312589