feat: add direct endpoint overrides for auxiliary and delegation

Add base_url/api_key overrides for auxiliary tasks and delegation so users can
route those flows straight to a custom OpenAI-compatible endpoint without
having to rely on provider=main or named custom providers.

Also clear gateway session env vars in test isolation so the full suite stays
deterministic when run from a messaging-backed agent session.

tools/delegate_tool.py

@@ -540,18 +540,51 @@ def delegate_task(
 def _resolve_delegation_credentials(cfg: dict, parent_agent) -> dict:
     """Resolve credentials for subagent delegation.
 
-    If ``delegation.provider`` is configured, resolves the full credential
-    bundle (base_url, api_key, api_mode, provider) via the runtime provider
-    system — the same path used by CLI/gateway startup.  This lets subagents
-    run on a completely different provider:model pair.
+    If ``delegation.base_url`` is configured, subagents use that direct
+    OpenAI-compatible endpoint. Otherwise, if ``delegation.provider`` is
+    configured, the full credential bundle (base_url, api_key, api_mode,
+    provider) is resolved via the runtime provider system — the same path used
+    by CLI/gateway startup. This lets subagents run on a completely different
+    provider:model pair.
 
-    If no provider is configured, returns None values so the child inherits
-    everything from the parent agent.
+    If neither base_url nor provider is configured, returns None values so the
+    child inherits everything from the parent agent.
 
     Raises ValueError with a user-friendly message on credential failure.
     """
-    configured_model = cfg.get("model") or None
-    configured_provider = cfg.get("provider") or None
+    configured_model = str(cfg.get("model") or "").strip() or None
+    configured_provider = str(cfg.get("provider") or "").strip() or None
+    configured_base_url = str(cfg.get("base_url") or "").strip() or None
+    configured_api_key = str(cfg.get("api_key") or "").strip() or None
+
+    if configured_base_url:
+        api_key = (
+            configured_api_key
+            or os.getenv("OPENAI_API_KEY", "").strip()
+        )
+        if not api_key:
+            raise ValueError(
+                "Delegation base_url is configured but no API key was found. "
+                "Set delegation.api_key or OPENAI_API_KEY."
+            )
+
+        base_lower = configured_base_url.lower()
+        provider = "custom"
+        api_mode = "chat_completions"
+        if "chatgpt.com/backend-api/codex" in base_lower:
+            provider = "openai-codex"
+            api_mode = "codex_responses"
+        elif "api.anthropic.com" in base_lower:
+            provider = "anthropic"
+            api_mode = "anthropic_messages"
+
+        return {
+            "model": configured_model,
+            "provider": provider,
+            "base_url": configured_base_url,
+            "api_key": api_key,
+            "api_mode": api_mode,
+        }
 
     if not configured_provider:
         # No provider override — child inherits everything from parent
@@ -570,7 +603,8 @@ def _resolve_delegation_credentials(cfg: dict, parent_agent) -> dict:
     except Exception as exc:
         raise ValueError(
             f"Cannot resolve delegation provider '{configured_provider}': {exc}. "
-            f"Check that the provider is configured (API key set, valid provider name). "
+            f"Check that the provider is configured (API key set, valid provider name), "
+            f"or set delegation.base_url/delegation.api_key for a direct endpoint. "
             f"Available providers: openrouter, nous, zai, kimi-coding, minimax."
         ) from exc