Support `@` imports in sub-agent markdown files

Memory files support an @ import syntax for importing other files.

Similar functionality in sub-agent files would be useful in various situations, such as:

• you have common guidance that you want to include in multiple agents
• you have knowledge you want the main agent to have, but a sub agent to enforce, and want to avoid duplication

Currently you can put @some/other/file.md in a sub-agent, and behaviourally it works as I would like, however, there are a few downsides that suggest to me that the @ import syntax isn't _really_ working, and instead Claude is just smart enough to read and process the file:

• It reads the imported file every time the agent is loaded, whereas agents usually get loaded once and then cached (proven by making edits directly to the agent's md file, as they don't take effect without a restart)
• It requires the Read tool

_Proper_ support for @ import should not require the Read tool, and should only import the file once when first loading the agent.