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/bug.md

@@ -0,0 +1,21 @@
+---
+name: Bug
+about: Something behaves incorrectly
+title: ""
+labels:
+  - type:bug
+---
+
+**What happened:**
+
+**What you expected:**
+
+**Steps to reproduce:**
+1.
+2.
+
+**Environment:** <!-- branch/commit, OS, container engine, anything relevant -->
+
+**Logs / evidence:**
+
+**Suspected area:** <!-- e.g. area:bff, area:acl — add the matching area label -->