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>

Makefile

@@ -19,7 +19,7 @@ export DOCKER_HOST := unix://$(PODMAN_SOCK)
 endif
 endif
 
-.PHONY: ci lint build unit smoke down help
+.PHONY: ci lint build unit smoke down changelog help
 
 ## ci: run the full pipeline — lint, build, unit, smoke (mirrors Gitea Actions)
 ci: lint build unit smoke
@@ -45,6 +45,10 @@ smoke:
 down:
 	docker compose -f $(COMPOSE) down --volumes
 
+## changelog: regenerate CHANGELOG.md from Conventional Commits (git-cliff)
+changelog:
+	git-cliff --output CHANGELOG.md
+
 ## help: list available targets
 help:
 	@grep -E '^## ' $(MAKEFILE_LIST) | sed 's/^## //'