fix: normalise Nous device-code pool source to avoid duplicates

Review feedback on the original commit: the helper wrote a pool entry
with source `manual:device_code` while `_seed_from_singletons()` upserts
with `device_code` (no `manual:` prefix), so the pool grew a duplicate
row on every `load_pool()` after login.

Normalise: the helper now writes `providers.nous` and delegates the pool
write entirely to `_seed_from_singletons()` via a follow-up
`load_pool()` call. The canonical source is `device_code`; the helper
never materialises a parallel `manual:device_code` entry.

- `persist_nous_credentials()` loses its `label` and `source` kwargs —
  both are now derived by the seed path from the singleton state.
- CLI and web dashboard call sites simplified accordingly.
- New test `test_persist_nous_credentials_idempotent_no_duplicate_pool_entries`
  asserts that two consecutive persists leave exactly one pool row and
  no stray `manual:` entries.
- Existing `test_auth_add_nous_oauth_persists_pool_entry` updated to
  assert the canonical source and single-entry invariant.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

hermes_cli/web_server.py

@@ -1444,13 +1444,8 @@ def _nous_poller(session_id: str) -> None:
             auth_state, min_key_ttl_seconds=300, timeout_seconds=15.0,
             force_refresh=False, force_mint=True,
         )
-        from agent.credential_pool import SOURCE_MANUAL
         from hermes_cli.auth import persist_nous_credentials
-        persist_nous_credentials(
-            full_state,
-            label="dashboard device_code",
-            source=f"{SOURCE_MANUAL}:dashboard_device_code",
-        )
+        persist_nous_credentials(full_state)
         with _oauth_sessions_lock:
             sess["status"] = "approved"
         _log.info("oauth/device: nous login completed (session=%s)", session_id)