Add pull request template (closes the community-profile gap issue templates can't)

[Laaaaksh]

What

Adds .github/PULL_REQUEST_TEMPLATE.md, closing the community-standards gap left after PR #24: CODE_OF_CONDUCT.md lifted health 71%→85%, but investigation this iteration proved GitHub's community/profile → files.issue_template field is dead GitHub-wide — cli/cli reaches 100% health with issue_template: null, as do pallets/flask (87%), gin-gonic/gin (75%), and rust-lang/rust despite canonical .md templates on disk. The YAML issue form from PR #24 remains fully valid for users (schema-checked, industry-standard); the metric simply no longer recognizes any template type.

The one community-profile item writ genuinely lacked is a pull request template (files.pull_request_template), which cli/cli's 100% profile shows is still live-counted. This repo requires PRs for every change under strict branch protection, so the template also earns its keep: it mirrors CONTRIBUTING.md's flow (what changed + the exact three-command local verification CI reruns) with a short checklist.

Validation

• Community-profile matrix surveyed across five flagship repos to isolate the dead metric from real gaps (details above)
• Pre-push sweep: gofmt -l clean, go build ./..., go vet ./..., go test ./... -race -count=1 green across all six packages