Add Feature

Use when adding a medium or larger feature (or a non-trivial change) to the azure-functions-agents-runtime repository — i.e. anything that introduces new public surface, spans multiple modules, or changes authoring/discovery/registration behavior. Drives the full feature lifecycle defined in AGENTS.md: triage + worktree, writing a Feature Requirements Document (FRD) with a Decisions log, an architecture-review checkpoint, surgical implementation, a testing checkpoint, and documentation updates.