Suggestion: Add a constitution-template.md to make upgrading Spec Kit easier

[DavidBurela]

Suggestion

Currently in .specify/templates there are templates for

• checklist-template.md
• plan-template.md
• spec-template.md
• tasks-template.md

I think we should add a new one for

• constitution-template.md

Justification

Currently when you upgrade spec kit, it will update the templates. Which is great, the next time you run /spec or /plan it will use the latest template to generate the new file.

However constitution.md does not follow this same pattern, of a template generating the first constitution.
This results in the constitution always getting overridden during upgrades

1. install spec kit, get the blank constitution.md
2. /speckit.constitution xxxx, to update it
3. upgrade spec kit, new blank constitution.md overwrites it

If the constitution-template.md existed, then upgrades would only overwrite the template, and the generated constitution will be left alone.
This would help solve the most common conflict upon upgrading.