ReleaseChecker + the changelog fetch were hardwired to the Gitea API path, so a
GitHub-hosted public repo would never resolve a release. Centralise the repo-URL
parsing and branch by host: GitHub uses api.github.com for tags and
raw.githubusercontent.com for CHANGELOG.md (with the User-Agent GitHub requires);
Gitea keeps its /api/v1 paths. The changelog fetch moves into ReleaseChecker so
all host logic lives in one place.
Make the update-check tests hermetic: they now set clusev.repository themselves
(neutral host) instead of depending on the ambient .env — so they pass in CI with
an empty .env.example — and add GitHub-host coverage for tags + changelog.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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