Function resolve 

pub fn resolve(
    config: &Config,
    name: &str,
    bundle: Option<&str>,
) -> Result<SkillRef, SkillRefError>

Resolve a (name, bundle?) pair into a canonical SkillRef.

Rule: bundle is optional iff name exists in exactly one configured bundle’s directory. Otherwise the caller must qualify.

Filesystem state (which directories actually contain a SKILL.md) is checked by crate::skills::service::SkillsService::list_skills; this function operates over Config alone and is filesystem-free, so it can be unit-tested in isolation.