chore: contributor workflow — issue/PR templates, git-cliff, gitea-workflow doc (refs #31)

Add the Gitea issue templates (slice/bug/adr-proposal) and a PR template
that carry the Definition of Done, a git-cliff config (cliff.toml) with a
`make changelog` target, the generated CHANGELOG.md, and docs/gitea-
workflow.md documenting the issue -> milestone -> PR flow.

Verified: make lint/build/unit green; `make changelog` regenerates the
changelog from Conventional Commits.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitea/ISSUE_TEMPLATE/slice.md

@@ -0,0 +1,31 @@
+---
+name: Slice (user story)
+about: A backlog slice — independently demoable, encodes the Definition of Done
+title: "S-NN · "
+labels:
+  - type:slice
+---
+
+**Outcome:** <!-- one sentence; user-visible if possible -->
+
+**Acceptance:**
+<!-- Gherkin scenarios or testable assertions -->
+-
+
+**Touches:** <!-- services and folders -->
+
+**Out of scope:** <!-- explicit non-goals -->
+
+## Definition of Done
+
+- [ ] This linked Gitea issue exists and is on the right milestone.
+- [ ] Failing test written and committed first (`test(scope): … (refs #NN)`).
+- [ ] Implementation makes the test pass (`feat(scope): … (refs #NN)`).
+- [ ] Refactor commit follows if structure improved.
+- [ ] Conventional Commit messages referencing this issue.
+- [ ] All Gitea Actions CI jobs green (or `make ci` green while no runner exists).
+- [ ] `docker compose up` from a fresh clone reaches green health checks within 3 minutes.
+- [ ] Docs touched if behaviour, contracts, or operations changed.
+- [ ] ADR added in `docs/architecture/` if a non-obvious decision was made.
+- [ ] Demo note appended to `docs/demo-script.md` if the slice is user-visible.
+- [ ] This issue closed by the merging PR (`closes #NN`).