Function read_capability_doc 

async fn read_capability_doc(
    call: &CapabilityCall,
    atlas: &mut AtlasClient,
) -> CapabilityCallResult

Apply the read_capability_doc builtin: fetch a provider’s registered CAPABILITY.md content from atlas and return it as markdown text.

Unlike read_file, this never touches the filesystem — atlas serves the content the provider sent at registration, so it works regardless of the provider’s (possibly containerised) mount layout. Errors when the provider is unknown or registered no doc; output is truncated to keep prompt size bounded.