Validate a skill name against the agentskills.io spec rules:
1-64 characters, lowercase a-z, digits, and hyphens only, must not
start/end with a hyphen and must not contain consecutive hyphens.
Returns Ok(()) on conformance, otherwise a human-readable diagnostic.
Used by skill-creation tooling so newly-authored skills are spec-clean
without rejecting any pre-existing locally installed skills.