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>
