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>

CHANGELOG.md

@@ -0,0 +1,20 @@
+# Changelog
+
+All notable changes to this project. Generated from Conventional Commits by git-cliff.
+
+## Unreleased
+
+### CI
+- Gitea Actions pipeline + runner runbook (refs #30) (#37)
+
+### Chores
+- Add idempotent Gitea backlog seeder
+- Remove bootstrap scripts from main (#35)
+
+### Documentation
+- Split S-00 into sub-slices (refs #1) (#33)
+
+### Features
+- Placeholder BFF + /health endpoint (closes #28) (#34)
+- Containerize BFF + compose-up smoke (closes #29) (#36)
+