docs: explicit tool list for Plan subagent is missing

Problem

The subagents documentation describes the Plan subagent's tool access as:

Tools: Read-only tools (denied access to Write and Edit tools)

This is ambiguous. It's unclear exactly which tools are available to the Plan subagent. Specifically:

• Is the Skill tool available? (A read-only skill failed to execute in a Plan subagent)
• Is Bash available?
• Is AskUserQuestion available?
• Is TaskOutput available?
• Are MCP tools available?

The Explore and general-purpose subagents have the same ambiguity, though general-purpose says "All tools" which is clearer.

Expected

An explicit, enumerated list of tools available to each built-in subagent (Plan, Explore, general-purpose, Bash, etc.), similar to how custom subagent docs show the tools field accepting specific tool names.

Context

Tried to invoke a skill (containing only read instructions) from a Plan subagent — it did not execute. Without a clear tool list, it's impossible to know if this is expected behavior or a bug.