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

Makefile

@@ -56,6 +56,7 @@ CORE_SRC = \
 	src/ctl_parse.c \
 	src/os_compat.c \
 	src/sandbox.c \
+	src/sandbox_exec.c \
 	src/session.c \
 	src/tools/tools.c \
 	src/tools/patch.c \
@@ -96,7 +97,9 @@ TESTS = \
 	tests/test_tools_fs \
 	tests/test_registry \
 	tests/test_undo \
-	tests/test_config
+	tests/test_config \
+	tests/test_models \
+	tests/test_sandbox
 
 TEST_LIBS = -lm $(CURL_LIBS) $(PTHREAD_LIBS)