Subagents: Limit read/write access to specific file globs

[digitarald]

Problem
When using runSubagent, subagents currently have unrestricted read/write access to the workspace. This makes it hard to safely delegate research or content-generation tasks without risking unintended edits.

Proposal
Add support for optional read/write scoping when invoking a subagent.

This would let subagents read the full workspace but only write to approved file globs (or none at all), improving safety and control in multi-agent workflows.

Use case
We run research subagents that build Markdown docs for later synthesis by other agents. They should only write inside their research directory.

cc @pierceboggan