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

include/os_compat.h

@@ -57,6 +57,16 @@ 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);
 
+/* Same, with child_hook(ud) run in the child just before exec (Unix only;
+ * ignored on Windows). A non-zero return aborts the child with exit 126. */
+typedef int (*os_child_hook_fn)(void *ud);
+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);
+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);
+
 /* Child processes get stdin=/dev/null, their own process group, and are
  * killed after the timeout (CST_SHELL_TIMEOUT seconds, default 120) or when
  * *flag becomes non-zero. Exit code 124 = timeout, 130 = cancelled. */