GitHub/spec-kit: Get started with Spec-Driven Development

How is this distinct from workflows that include README.md, AGENTS.md or .agents/, and subagents?

"AGENTS.md – Open format for guiding coding agents": https://news.ycombinator.com/item?id=44957443

How does the proposed software development process differ from Formal Methods (i.e. Formal Specification, Implementation, and Verification)?