docs(sdk): explain inherited subagent capability limits

[Wrestle7]

• I have read and reviewed the documentation changes to the best of my ability.
• If the change is significant, I have run the documentation site locally and confirmed it renders as expected.

Summary of changes

Problem

The task-tool guide needs to explain the explicit subagent tool/MCP ceilings proposed for OpenHands/software-agent-sdk#4953 and how callers opt in when constructing an Agent directly.

Solution

Add the capability-selection semantics and a concrete Agent configuration example next to the task tool workflow.

Changes

• Explain independent unrestricted, empty, and subset selections for tools and MCP.
• Show direct Agent configuration with SubagentCapabilityLimits.from_selection.
• Describe nested delegation, plugin checks, persistence/fork, and strict configuration matching.
• State the trusted-Python and external-tool boundaries of this mechanism.

Testing

• Reviewed the MDX diff and checked whitespace and balanced code fences.
• Executed the new Python example against the companion SDK implementation without external model/API calls.
• Mintlify site rendering/build was not run; the site-preview checkbox is intentionally unchecked.

Notes for Reviewer

Companion implementation: OpenHands/software-agent-sdk#5365

The SDK draft explicitly overlaps OpenHands/software-agent-sdk#5358 and adds MCP and other delegation/lifecycle paths. These docs describe its proposed strict-rejection behavior; keep this draft until maintainers settle the implementation and the SDK change is available. AI-assisted documentation update.