Growth: Add GitHub topics for discoverability

## What

Add topics to the repo so it shows up in searches.

## Suggested Topics

`rust`, `cli`, `data-format`, `json`, `yaml`, `toml`, `csv`, `xml`, `data-converter`, `data-transformation`, `command-line-tool`, `serde`, `messagepack`, `jsonl`, `hacktoberfest`

## How

Repo → Settings → Topics

## 👤 Human-in-the-loop

Quick admin settings change — no blockers, do anytime.