feat: Add disable native subagents config option

[georgecollier-nqu]

CLIs like Claude Code have their own subagent support.

Sometimes, the agent will use these subagents. In bb, all agents should use the CLI to orchestrate subagents. There should be a setting for Claude Code/Codex etc which lets you easily disable/enable native subagents. This is currently possible to workaround just by modifying the Claude Code/Codex configs but a UI would make it a lot more straight forward.

[SawyerHood]

This is a banger feature! We at one point do this and had a full middle manager feature like https://github.com/SawyerHood/middleman, but we tore it out to make the product a bit more focused.

Feel free to open a PR for this! If not I'll try to pick it up sometime next week.

My thought is:

• Toggle in settings default off that is "Disable provider subagents"
• This disables the Task tool in Claude Code and whatever the codex app server equivalent is.

In the near future I want to expose more "agent configuration" so you can basically pick a harness, add and remove tools and skills, change the prompt so you can build your own custom agents. Ex if you want to build a chief of staff / middle man you could sculpt one.

But for now the setting is fine :)