Function verify_spec 

pub fn verify_spec(
    spec: &Spec,
    config: &Config,
    options: VerifyOptions,
    invoke_agent_fn: impl Fn(&str, &Spec, &str, &Config) -> Result<String>,
) -> Result<(VerificationStatus, Vec<CriterionResult>)>

Verify a spec by invoking the agent.

This is the canonical verification logic:

• Checks for acceptance criteria
• Assembles verification prompt
• Invokes the agent
• Parses verification response
• Updates spec frontmatter

Returns (overall_status, criteria_results).