adr: smaller fix for the keyless-harness model gate

adrs/model-provider-gate-should-ask-the-harness.md

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+# Smaller fix for the keyless-harness gate
+
+Same thing #67 hit. I'm on `HARNESS=claude` with a `CLAUDE_CODE_OAUTH_TOKEN`, no API key, Postgres behind it. Admins
+get redirected to `/admin/onboarding` on every request and everyone else gets the 503 about the deployment not being
+set up. Turns work the whole time, it's just the gate.
+
+Different suggestion for where to fix it though.
+
+There's already a function for this. `modelProviderAvailabilityFor` in `src/model/pi-models.ts:213` takes a harness and
+gives back the providers that harness can use, and `claude` falls through to `ALL_PROVIDERS_AVAILABLE`:
+
+```ts
+if (harness === "pi") return managedKeys;
+if (harness === "opencode") return { ...configKeys, openrouter: false };
+if (harness === "codex") return configKeys;
+return ALL_PROVIDERS_AVAILABLE; // claude and mock
+```
+
+`surface.ts` imports it and uses it at line 933 for the runtime config. Line 892, the onboarding flag, doesn't:
+
+```ts
+...(managedKeys ? { modelProviderConfigured: Object.values(managedKeys).some(Boolean) } : {}),
+```
+
+So the same file works it out two different ways. `git log -S` puts line 892 in #29, which was about routing the first
+admin to onboarding, so I'd guess the harness case just never came up.
+
+What I've got running:
+
+```ts
+modelProviderConfigured: Object.values(modelProviderAvailabilityFor(harnessId, managedKeys)).some(Boolean),
+```
+
+`harnessId` is already computed in `getSurfaceConfig` and the helper is already imported, so it's the one line. `pi`
+and `codex` are unchanged, `claude` and `mock` no longer need a key stored, and `opencode` only shifts if the one
+stored key is OpenRouter, which it can't use anyway.
+
+Versus the wiring.ts wrapper in #67: no need to go looking for `CLAUDE_CODE_OAUTH_TOKEN`, and `mock` gets sorted too.
+Downside is that `claude` reports configured even with no token set anywhere, so a deploy that really is broken gets a
+working UI and failing turns instead of the onboarding page. #67 doesn't have that problem. I'd still take it, because
+the onboarding page can't tell you anything useful about OAuth auth either way, but it's a real difference and worth
+deciding on before picking one.
+
+Typecheck's clean, `test/model-credential-route.test.ts` is 11/11 and the `surface*` tests are 126/126. Applied it on
+our own deployment earlier today. If you want a regression test it'd go in the core surface-config tests, flag true
+for `harnessId: "claude"` against an empty credential store.
+`plugins/portal/test/onboarding-redirect.test.ts` builds the response by hand so it never gets near this.