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>

cliff.toml

@@ -0,0 +1,45 @@
+# git-cliff configuration — generates CHANGELOG.md from Conventional Commits.
+# Run via `make changelog`. See https://git-cliff.org.
+
+[changelog]
+header = """
+# Changelog
+
+All notable changes to this project. Generated from Conventional Commits by git-cliff.\n
+"""
+body = """
+{% if version %}\
+## {{ version }} — {{ timestamp | date(format="%Y-%m-%d") }}
+{% else %}\
+## Unreleased
+{% endif %}\
+{% for group, commits in commits | group_by(attribute="group") %}
+### {{ group | upper_first }}
+{% for commit in commits %}\
+- {{ commit.message | upper_first }}{% if commit.breaking %} **[BREAKING]**{% endif %}
+{% endfor %}\
+{% endfor %}\n
+"""
+trim = true
+
+[git]
+conventional_commits = true
+filter_unconventional = true
+split_commits = false
+protect_breaking_commits = true
+tag_pattern = "v[0-9]*"
+# CalVer tags: YYYY.MM.PATCH
+filter_commits = false
+commit_parsers = [
+  { message = "^feat", group = "Features" },
+  { message = "^fix", group = "Bug Fixes" },
+  { message = "^perf", group = "Performance" },
+  { message = "^refactor", group = "Refactor" },
+  { message = "^docs", group = "Documentation" },
+  { message = "^test", group = "Tests" },
+  { message = "^ci", group = "CI" },
+  { message = "^build", group = "Build" },
+  { message = "^arch", group = "Architecture" },
+  { message = "^chore", group = "Chores" },
+  { message = ".*", group = "Other" },
+]