docs: add pull request template

[LouisDeconinck]

Summary

Add a short pull request template covering the summary, linked issue, tests, and DCO sign-off.

Checks

• git diff --check

Closes #142

[github-actions]

Thanks for your first pull request to Sotto, and welcome.

Two things that trip up newcomers:

• Every commit needs a Signed-off-by line under the Developer Certificate of Origin - PRs with unsigned commits cannot be merged. Sign off with git commit -s; if your branch already has unsigned commits, run git rebase --signoff main and push again.
• Please run cargo fmt --all --check and cargo clippy --workspace --all-targets -- -D warnings before pushing, and add tests for new behaviour (cargo test --workspace runs the suite).

The full guide is in CONTRIBUTING.md - a maintainer will review shortly.

[Maxerns]

Thanks for these contributions! CI is now green on #210–#214, but the branches are behind main, which blocks merging under our repository rules.
Could you enable “Allow edits from maintainers” in the right-hand sidebar on each PR? It may appear as “Allow edits and access to secrets by maintainers.” That would let me update the branches and handle the remaining merge steps.
If you’d prefer to keep that disabled, updating each PR branch with the latest getsotto/sotto main works too. @LouisDeconinck