This skill should be used when the user says "feature spec", "arness code feature spec", "arn-code-feature-spec", "spec this feature", "help me spec", "design this feature", "feature design", "write a spec", "create a specification", "I have an idea for a feature", "let's flesh out this feature", "decompose feature", "spec XL feature", "resume spec", "continue spec", "finish my spec", "break down feature", or wants to iteratively develop a feature idea into a well-formed specification through guided conversation with architectural analysis. For XL features with decomposition hints, creates multiple sub-feature specs with full traceability. Produces specification documents capturing WHAT to build and WHY, which then feed into plan creation.