[Feature Request] Subagents should inherit parent agent context to avoid redundant codebase scanning

Hey it would be really awesome if claude could spawn its subagents with the main agent's existing context. That way the subagents don't need to go around scanning the codebase for knowledge the main agent already has. Also means we won't have to do an expensive compact/compress to feed instructions and context into every agent.

This should be a frontmatter setting in each agent. Some agents would greatly benefit from this, for example test runner agents that prevent the main agent's context from being filled with junk.