A fallback chain entry can name its API key via key_env (or the
api_key_env alias) per the fallback-providers docs, but only the gateway
path resolved it — TUI/desktop, cron, and CLI setup fallbacks ignored it,
so a fallback provider whose key lives in a non-standard env var never
resolved on those surfaces.
Centralize the inline-api_key-then-key_env lookup in
hermes_cli/fallback_config.resolve_entry_api_key() and use it at all four
fallback resolution sites (tui_gateway, cron scheduler, gateway runner,
CLI setup mixin); the CLI mixin also gains the base_url passthrough the
other surfaces already had.
Salvaged from PR #43861 (surgical reapply — the original branch predates
the #65264 fallback restructuring).
