OpenCode-style TUI, model probing, sandboxed auto approval

- TUI: centered home screen with logo, boxed input with mode line, sidebar
  (>=120 cols), Ctrl+P palette, Tab Build/Plan, Shift+Tab approval cycle,
  mouse (cards, cursor, pickers, wheel), /sidebar /mouse /approval
- /models probing: key check on entry (401 re-asks), API models in /model
- Shell sandbox in C: Linux Landlock (writes limited to workspace/tmp,
  ABI>=4 blocks TCP), macOS sandbox_init, rules fallback elsewhere
- Approval modes ask/auto/yolo (default auto); read-only command whitelist;
  ask to rerun outside the sandbox when a command looks blocked
- Upgradable bootstrap scripts (git pull --ff-only, rebuild, reinstall) and
  cst-agent-update; cst-agent --version
- tests: test_sandbox, test_models

src/llm_openai.c

@@ -698,3 +698,115 @@ finish:
   free(body);
   return ret;
 }
+
+/* --- model listing -------------------------------------------------------- */
+
+#define MODELS_BODY_MAX (2 * 1024 * 1024)
+
+static size_t models_write_cb(char *ptr, size_t size, size_t nmemb, void *ud) {
+  cst_buf_t *b = ud;
+  size_t n = size * nmemb;
+  if (b->len + n > MODELS_BODY_MAX || cst_buf_append(b, ptr, n) != 0)
+    return 0;
+  return n;
+}
+
+static int add_model(char out[][128], int n, int max_out, const char *id) {
+  if (!id || !id[0] || n >= max_out || strlen(id) >= 128)
+    return n;
+  for (int i = 0; i < n; i++)
+    if (strcmp(out[i], id) == 0)
+      return n;
+  snprintf(out[n], 128, "%s", id);
+  return n + 1;
+}
+
+/* {"data":[{"id":..}]}, {"models":[{"id"|"name":..}]} or a bare array. */
+static int parse_models(const char *json, char out[][128], int max_out) {
+  cJSON *root = cJSON_Parse(json);
+  if (!root)
+    return -1;
+  const cJSON *arr = root;
+  if (!cJSON_IsArray(arr))
+    arr = cJSON_GetObjectItemCaseSensitive(root, "data");
+  if (!cJSON_IsArray(arr))
+    arr = cJSON_GetObjectItemCaseSensitive(root, "models");
+  int n = -1;
+  if (cJSON_IsArray(arr)) {
+    n = 0;
+    const cJSON *it;
+    cJSON_ArrayForEach(it, arr) {
+      const cJSON *id = cJSON_GetObjectItemCaseSensitive(it, "id");
+      if (!cJSON_IsString(id))
+        id = cJSON_GetObjectItemCaseSensitive(it, "name");
+      if (cJSON_IsString(id))
+        n = add_model(out, n, max_out, id->valuestring);
+      else if (cJSON_IsString(it))
+        n = add_model(out, n, max_out, it->valuestring);
+    }
+  }
+  cJSON_Delete(root);
+  return n;
+}
+
+int cst_llm_list_models(const char *api_base, const char *api_key,
+                        char out[][128], int max_out, char *err,
+                        size_t err_sz) {
+  char dummy[8];
+  if (!err || !err_sz) {
+    err = dummy;
+    err_sz = sizeof(dummy);
+  }
+  err[0] = 0;
+  if (!api_base || !api_base[0] || !out || max_out <= 0) {
+    snprintf(err, err_sz, "bad request");
+    return -1;
+  }
+  CURL *curl = curl_easy_init();
+  if (!curl) {
+    snprintf(err, err_sz, "curl init failed");
+    return -1;
+  }
+  char url[1024];
+  size_t bl = strlen(api_base);
+  while (bl && api_base[bl - 1] == '/')
+    bl--;
+  snprintf(url, sizeof(url), "%.*s/models", (int)bl, api_base);
+  struct curl_slist *hdrs = curl_slist_append(NULL, "Accept: application/json");
+  char auth[700];
+  if (api_key && api_key[0]) {
+    snprintf(auth, sizeof(auth), "Authorization: Bearer %s", api_key);
+    hdrs = curl_slist_append(hdrs, auth);
+  }
+  cst_buf_t body = {0};
+  curl_easy_setopt(curl, CURLOPT_URL, url);
+  curl_easy_setopt(curl, CURLOPT_HTTPHEADER, hdrs);
+  curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, models_write_cb);
+  curl_easy_setopt(curl, CURLOPT_WRITEDATA, &body);
+  curl_easy_setopt(curl, CURLOPT_CONNECTTIMEOUT, 10L);
+  curl_easy_setopt(curl, CURLOPT_TIMEOUT, 10L);
+  curl_easy_setopt(curl, CURLOPT_NOSIGNAL, 1L);
+  curl_easy_setopt(curl, CURLOPT_FOLLOWLOCATION, 1L);
+  CURLcode rc = curl_easy_perform(curl);
+  long http = 0;
+  curl_easy_getinfo(curl, CURLINFO_RESPONSE_CODE, &http);
+  curl_slist_free_all(hdrs);
+  curl_easy_cleanup(curl);
+  memset(auth, 0, sizeof(auth));
+
+  int n = -1;
+  if (rc != CURLE_OK) {
+    snprintf(err, err_sz, "curl: %s", curl_easy_strerror(rc));
+  } else if (http == 401 || http == 403) {
+    snprintf(err, err_sz, "HTTP %ld: the API key was rejected", http);
+    n = CST_MODELS_AUTH;
+  } else if (http >= 400) {
+    snprintf(err, err_sz, "HTTP %ld from %s", http, url);
+  } else {
+    n = parse_models(body.buf ? body.buf : "", out, max_out);
+    if (n < 0)
+      snprintf(err, err_sz, "unexpected /models response");
+  }
+  cst_buf_free(&body);
+  return n;
+}