chore: contributor workflow — templates, git-cliff, gitea-workflow doc (closes #31) (#38)

.gitea/PULL_REQUEST_TEMPLATE.md

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+<!-- Title: Conventional Commit style, e.g. feat(bff): … (closes #NN) -->
+
+## What & why
+
+<!-- Summary of the change and the slice/bug it addresses. -->
+
+Closes #
+
+## Definition of Done
+
+- [ ] Linked Gitea issue (above).
+- [ ] Failing test committed before the implementation.
+- [ ] Implementation makes the test pass; refactor commit if structure improved.
+- [ ] Conventional Commits referencing the issue (`refs #NN`).
+- [ ] CI green — all Gitea Actions jobs (or `make ci` green while no runner exists).
+- [ ] `docker compose up` from a fresh clone reaches green health checks within 3 minutes.
+- [ ] Docs updated if behaviour, contracts, or operations changed.
+- [ ] ADR added in `docs/architecture/` if a non-obvious decision was made.
+- [ ] Demo note in `docs/demo-script.md` if user-visible.
+
+## Notes for reviewers
+
+<!-- Anything that helps review: trade-offs, follow-ups, known gaps. -->