Phase 1: Parse Arguments

Determine whether this is a sprint retrospective (sprint-N) or a milestone retrospective (milestone-name).

Phase 1b: Check for Existing Retrospective

Before loading any data, glob for an existing retrospective file:

For sprint retrospectives: production/retrospectives/retro-[sprint-slug]-*.md (also check production/sprints/sprint-[N]-retrospective.md as an alternate location)
For milestone retrospectives: production/retrospectives/retro-[milestone-name]-*.md

If a matching file is found, use AskUserQuestion:

Prompt: "An existing retrospective was found: [filename]. How do you want to proceed?"
Options:
- [A] Update existing — load it and add/revise sections with new data
- [B] Start fresh — generate a new retrospective (archive the old one)