Add a pull request template

[Glittersup]

Summary

Pull requests arrive in inconsistent shapes, some without a linked issue or any note of what was tested. A template prompts for both without adding process.

Acceptance criteria

• A pull request template asks for a summary, the linked issue and what was tested.
• It includes a short checklist covering the local checks.
• GitHub applies it to new pull requests.

Getting started

Fork this repository, clone your fork, and add this repo as upstream:

git clone https://github.com/<your-username>/tricklepay-backend.git
cd tricklepay-backend
git remote add upstream https://github.com/TricklePay/tricklepay-backend.git

Create a branch for this issue:

git checkout -b chore/issue-169

Suggested commit message:

chore: add a pull request template

Run npm run typecheck, npm test, and npm run build before opening a pull request and linking this issue.