docs+test: custom providers in README and AGENTS.md, provider-only startup PTY test

Also records the codex catalog at eight models and the 0.159.0 User-Agent.

crates/codegen/kigi-tui/tests/pty_e2e/custom_provider.rs

@@ -127,3 +127,38 @@ async fn custom_anthropic_provider_uses_messages_and_x_api_key() {
     }
     harness.quit().expect("clean quit");
 }
+
+/// A saved provider is a complete login: with no house key and no session, the
+/// pager starts authenticated and the turn reaches the provider.
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+#[ignore]
+async fn custom_provider_alone_authenticates_at_startup() {
+    let content = ContentController::start_with_models(vec![MockModel::new("custom-m1")])
+        .await
+        .expect("start content");
+    declare_provider(&content, "openai");
+    content.set_response(format!(
+        "{MOCK_RESPONSE_SENTINEL} from the custom provider."
+    ));
+    let env: Vec<(String, String)> = content
+        .env_for_pager()
+        .into_iter()
+        .filter(|(name, _)| name != "KIGI_API_KEY")
+        .collect();
+    let env_refs: Vec<(&str, &str)> = env.iter().map(|(k, v)| (k.as_str(), v.as_str())).collect();
+    let binary = pager_binary().expect("resolve pager binary");
+    let mut harness =
+        PtyHarness::new_in_dir(&binary, DEFAULT_ROWS, DEFAULT_COLS, &[], &env_refs, None)
+            .expect("spawn pager without a house key");
+
+    harness
+        .wait_for_text(WELCOME_SCREEN_SENTINEL, WELCOME_TIMEOUT)
+        .expect("welcome text");
+    harness
+        .inject_keys(format!("{PROMPT}\r").as_bytes())
+        .expect("submit prompt");
+    harness
+        .wait_for_text(MOCK_RESPONSE_SENTINEL, Duration::from_secs(30))
+        .expect("the turn reaches the custom provider without a login screen");
+    harness.quit().expect("clean quit");
+}