fix(auxiliary): auto never bills a provider the user did not select

With a main provider selected, an unusable main route (expired xAI/Codex
OAuth token, 401/402/429 mid-session) fell through the built-in discovery
chain (OpenRouter -> Nous -> custom -> api-key) and quietly ran every
compression, title and memory-flush call on whichever OTHER account was
still logged in. Reported as "using Grok on my Premium+ sub, my Nous
Portal balance kept draining" — the chat visibly stayed on Grok while the
side tasks were billed elsewhere, and re-logging into X did not help
because the aux side never consulted the selected provider.

The discovery chain is now reserved for installs with no selected main
provider (`model.provider: auto` / unset). Otherwise the ladder is
main -> auxiliary.<task>.fallback_chain -> fallback_providers -> refuse
with a warning naming the dead provider and the fix. Both entry points
gate on the same predicate: the resolve-time route and the mid-request
payment/auth hop (_try_payment_fallback).

Existing chain tests that asserted the hop now pin `provider=auto`, the
one case where discovery is still the contract.

agent/auxiliary_client.py

@@ -3827,6 +3827,8 @@ def _try_payment_fallback(
     provider (and the main-provider path when it maps to the same backend). Returns (client, model, label) or (None, None, "")."""
     skip = failed_provider.lower().strip()
     main_provider = _read_main_provider()
+    if not _discovery_chain_allowed(main_provider, task):
+        return None, None, ""
     skip_labels = {skip}
     if main_provider and main_provider.lower() in skip:
         skip_labels.add(main_provider.lower())
@@ -4200,6 +4202,21 @@ def _try_main_provider_route(
     return client, resolved or main_model, resolved_provider
 
 
+def _discovery_chain_allowed(main_provider: str, task: Optional[str] = None) -> bool:
+    """The built-in discovery chain is a convenience for installs with NO selected main provider.
+    Once the user picked one, every auxiliary route must be a provider they configured (main,
+    ``auxiliary.<task>``, ``fallback_providers``); guessing "whatever else is logged in" bills an
+    account they never pointed this session at (xAI OAuth session with a dead token → every
+    compression silently charged to a Nous Portal balance)."""
+    if (main_provider or "").strip().lower() in {"", "auto"}:
+        return True
+    logger.warning(
+        "Auxiliary %s: main provider %s is unavailable and no fallback_chain / fallback_providers is "
+        "configured — refusing to guess another logged-in provider. Re-authenticate (`hermes model`) "
+        "or declare a fallback.", task or "call", main_provider)
+    return False
+
+
 def _try_discovery_chain() -> Tuple[Optional[OpenAI], Optional[str], str]:
     """Step 3: hardcoded aggregator/fallback chain, skipping unhealthy providers."""
     tried = []
@@ -4249,6 +4266,8 @@ def _resolve_auto_route(
         task, main_provider or "auto", reason="main provider unavailable")
     if fb_client is not None:
         return fb_client, fb_model, fb_label
+    if not _discovery_chain_allowed(main_provider, task):
+        return None, None, ""
     return _try_discovery_chain()