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

CMakeLists.txt

@@ -50,6 +50,7 @@ set(CST_CORE_SOURCES
   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
@@ -166,7 +167,7 @@ endforeach()
 # Tests that link the whole core (no TUI).
 foreach(t test_llm_body test_sse_decode test_apply_patch test_session_roundtrip
           test_shell test_diff test_tools_fs test_registry
-          test_undo test_config)
+          test_undo test_config test_models test_sandbox)
   add_executable(${t} tests/${t}.c ${CST_CORE_SOURCES})
   target_include_directories(${t} PRIVATE include "${CJSON_DIR}")
   target_compile_definitions(${t} PRIVATE _GNU_SOURCE)