persist_nous_credentials() now accepts an optional label kwarg which
gets embedded in providers.nous under the 'label' key.
_seed_from_singletons() prefers the embedded label over the
auto-derived label_from_token() fingerprint when materialising the
pool entry, so re-seeding on every load_pool('nous') preserves the
user's chosen label.
auth_commands.py threads --label through to the helper, restoring
parity with how other OAuth providers (anthropic, codex, google,
qwen) honor the flag.
Tests: 4 new (embed, reseed-survives, no-label fallback, end-to-end
through auth_add_command). All 390 nous/auth/credential_pool tests
pass.
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