[FEATURE] Subagents have no Agent/Task tool — recursive/nested subagent dispatch impossible

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

## Summary
Subagents dispatched via the Agent (Task) tool do not themselves
receive the Agent tool, so they cannot spawn their own subagents.
Recursive/nested orchestration (depth > 1) is impossible — all
fan-out must originate from the single top-level session.

## Expected
A subagent can invoke the Agent tool to dispatch a child subagent
(ideally with a configurable depth cap to prevent fork-bombs).

## Actual
A dispatched general-purpose agent reports the Agent/Task tool is
absent from its toolset; a ToolSearch for it returns only
TaskCreate (a todo manager). It cannot spawn a child.

## Why it matters
Blocks recursive divide-and-conquer patterns where each work unit is
itself an orchestratable sub-pipeline. Forces flat one-level fan-out
and keeps all orchestration/context in the root session.

Proposed Solution

Expose the Agent tool to subagents behind a depth limit, OR document
the limitation and provide a supported bounded-recursion primitive.

Alternative Solutions

_No response_

Priority

Medium - Would be very helpful

Feature Category

Configuration and settings

Use Case Example

1. Dispatch a general-purpose agent.
2. Instruct it to call the Agent tool to spawn one subagent and relay

the result.

1. It returns: NESTED_SPAWN: TOOL_UNAVAILABLE.

Additional Context

_No response_