add-to-glossary
Adds a new entry to docs/GLOSSARY.md:
- Placed under the correct A–Z letter section.
- Level-3 heading (
### Term Name) with kebab-case anchor. - 2–4 sentence definition.
- Ends with
**Reference:** [title](url)— required.
Replaces the section's _No entries yet._ placeholder if the section
was empty. Refuses to add duplicates.