This adds a returnRunResult option to agent.asTool() that allows developers to access the full RunResult of the sub-agent when invoked as a tool.

By default, asTool() only returns the final output string of the sub-agent, which means any interruptions (e.g. approval requests via needsApproval) are hidden from the parent agent.

With this flag enabled:

• The full RunResult is returned
• Interruptions are visible to the outer agent
• Human-in-the-loop and nested approval workflows now work as expected

This change is backward-compatible and opt-in. It does not affect any existing users of asTool().

Example usage:

agentB.asTool({
  toolName: 'agentB',
  returnRunResult: true,
});

Resolves #243