implementing-features

@tomgun

Implements features using contract-driven workflow with structural gates. Use when user says "build", "implement", "add feature", "create [thing]", "implement F-XXXX", "ag implement", or describes new functionality to build. Do NOT use for: one-line fixes (use fixing-bugs), writing tests only (use writing-tests), code review (use reviewing-code), documentation-only changes (use updating-documentation).