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/os_compat.c

@@ -588,9 +588,23 @@ int os_run_capture(const char *cwd, const char *const argv[], size_t max_out,
   r->exit_code = (int)code;
   return 0;
 }
+
+int os_run_capture_ex(const char *cwd, const char *const argv[],
+                      size_t max_out, os_run_result_t *r,
+                      os_child_hook_fn child_hook, void *ud) {
+  (void)child_hook;
+  (void)ud;
+  return os_run_capture(cwd, argv, max_out, r);
+}
 #else
 int os_run_capture(const char *cwd, const char *const argv[], size_t max_out,
                    os_run_result_t *r) {
+  return os_run_capture_ex(cwd, argv, max_out, r, NULL, NULL);
+}
+
+int os_run_capture_ex(const char *cwd, const char *const argv[],
+                      size_t max_out, os_run_result_t *r,
+                      os_child_hook_fn child_hook, void *ud) {
   if (!argv || !argv[0] || !r)
     return -1;
   if (max_out == 0)
@@ -619,6 +633,13 @@ int os_run_capture(const char *cwd, const char *const argv[], size_t max_out,
     close(pipefd[1]);
     if (cwd && cwd[0] && chdir(cwd) != 0)
       _exit(127);
+    if (child_hook && child_hook(ud) != 0) {
+      static const char msg[] = "[cst: sandbox setup failed; command not "
+                                "run]\n";
+      ssize_t w = write(STDERR_FILENO, msg, sizeof(msg) - 1);
+      (void)w;
+      _exit(126);
+    }
     execvp(argv[0], (char *const *)argv);
     _exit(127);
   }
@@ -707,6 +728,12 @@ int os_run_capture(const char *cwd, const char *const argv[], size_t max_out,
 
 int os_run_shell_c(const char *cwd, const char *cmd, size_t max_out,
                    os_run_result_t *r) {
+  return os_run_shell_ex(cwd, cmd, max_out, r, NULL, NULL);
+}
+
+int os_run_shell_ex(const char *cwd, const char *cmd, size_t max_out,
+                    os_run_result_t *r, os_child_hook_fn child_hook,
+                    void *ud) {
   if (!cmd || !r)
     return -1;
   char shell[OS_PATH_MAX];
@@ -731,10 +758,10 @@ int os_run_shell_c(const char *cwd, const char *cmd, size_t max_out,
 
   if (is_busybox) {
     const char *argv[] = {shell, "sh", "-c", cmd, NULL};
-    return os_run_capture(cwd, argv, max_out, r);
+    return os_run_capture_ex(cwd, argv, max_out, r, child_hook, ud);
   } else {
     const char *argv[] = {shell, "-c", cmd, NULL};
-    return os_run_capture(cwd, argv, max_out, r);
+    return os_run_capture_ex(cwd, argv, max_out, r, child_hook, ud);
   }
 }