[Feature Request] Support rules import in subagent profiles

Feature Request

Support @rule.md import in agents/{agent}.md files, the same as the mechanism in CLAUDE.md.

Current Behaviour

Currently you can only explicitly ask the agent to read certain files in the system prompt of the subagent definition files, which is not robust. For example, if you ask the subagent to output the rules verbatim, it will only include the rules in CLAUDE.md and its imports.