Add a pull request template

[nojibe]

Problem

There is no .github/PULL_REQUEST_TEMPLATE.md. PRs arrive without a consistent description, test plan, or linked issue, which slows review.

Proposed change

Add .github/PULL_REQUEST_TEMPLATE.md with sections mirroring the repo's existing pr-describe skill:

• Summary
• Changes
• Test plan (which of typecheck/lint/test:web/test:cli/test:e2e were run)
• Screenshots (for UI changes)
• Related issues (Closes #)

Acceptance criteria

• Template renders automatically when opening a PR
• Includes a checklist referencing the local quality gates (pnpm typecheck, pnpm lint, pnpm test)
• Prompts for a linked issue