Problem: adding a new forge required touching 5 lookup tables
(`FORGE_HOSTS`, `FORGE_CLI`, `FORGE_AUTH_CMD`, `SHORTHAND_PREFIX`,
`_warned_forges`) and every branching site in `_api_args`,
`fetch_metadata`, and `parse_ref`.
Solution: introduce a `ForgeBackend` class with `parse_url`,
`api_args`, and `parse_state` methods, plus a `register()` /
`backends()` registry. New forges (Gitea, Forgejo) are a single
`register()` call via the `gitea_backend()` convenience constructor.
Barrett Ruth
0033b26e38