~/.cert/deepseek.key is a DeepSeek-only credential; when a models profile
specified its own api_key_env (e.g. MOONSHOT_API_KEY) but the env var was
unset, resolve_api_key silently read that file and sent the DeepSeek key
as Bearer to a third-party endpoint (cross-vendor credential leak).
resolve_api_key gains allow_key_file (default True for the legacy flat
api section); create_llm_client passes allow_key_file=profile is None so
any profile path raises a clear RuntimeError instead, pointing users to
set the env var or use "" for local keyless endpoints.
Tests: new TestH1KeyIsolation (4 cases) with a mocked readable
deepseek.key proving the profile path errors out while the flat api
section still reads the file; full suite 276 passed.
qing