Windows: official port -- UCRT static build, UTF-8 throughout, Low IL sandbox, ncurses TUI, installer; v0.6.0

Build
  * cross build switches to pkgsCross.ucrt64 (shell-win.nix); curl (Schannel),
    winpthreads and ncurses 6.6 are linked statically, so the release is
    cst-agent.exe + busybox.exe (busybox-w32 Unicode) with no extra DLLs
  * windres embeds a manifest with activeCodePage=UTF-8 and a version resource
  * ncurses 6.6 Windows console driver patches (mouse button state, A_DIM,
    wide-char declarations): scripts/patches/
  * scripts/bootstrap_msys2.sh builds the same release natively in MSYS2
    UCRT64; PR #1's MINGW64 notes now point there
  * vendor/tre-regex (musl's TRE) provides <regex.h> on MinGW

UTF-8
  * paths cross the system call boundary as UTF-16 (*W APIs); console code
    pages set to 65001 and restored on exit; files are read and written in
    binary mode
  * child output that is not valid UTF-8 is decoded from the console, OEM or
    ANSI system code page
  * read_file hides CR; edit_file, apply_patch and write_file keep CRLF line
    endings and a UTF-8 BOM

OS layer and processes
  * os_dir_* and cst_fnmatch replace the POSIX-only directory walks in
    subagent, plugins, skills, undo and fs_tools; realpath / path_under and
    symlink rejection work on Windows
  * every child goes through CreateProcessW with its own environment block
    inside a Job Object, so timeouts and Esc end the whole process tree

Sandbox (auto mode, "auto · lowil")
  * run_shell runs with a restricted Low integrity token; the workspace gets
    an inheritable Low label once, the sandbox TEMP is labeled Low, and the
    credentials file gets a no-read-up label
  * drive roots, the profile and system folders are never labeled
  * network commands still ask, since Low IL does not block the network;
    Access denied and MSYS2's 0xc0000142 offer a rerun outside the sandbox
  * /sandbox lists labeled folders, /sandbox reset-labels restores them

TUI
  * raster console fonts are switched to Consolas / NSimSun while running
  * the home logo falls back to text when the console font lacks U+2580
  * mouse drag copy goes to the Windows clipboard

Distribution
  * scripts/install_windows.ps1 installs a Gitea release into
    %LOCALAPPDATA%\Programs\cst-agent and adds it to the user PATH;
    cst-agent-update.cmd reruns it; scripts/release_windows.sh uploads the
    zip with the token taken from the environment
  * scripts/run_windows.bat is gone; docs/README-windows.md is rewritten

Tests
  * new test_lowil, test_win_utf8 and test_os_dir; tests made portable
    (cst_test_util.h)
  * scripts/win_test.sh and scripts/winvm/ run the suite on English and
    Chinese Windows Server VMs as Administrator, a standard user and a
    Chinese user name: 29/29 on all six

tests/test_plugins.c

@@ -14,6 +14,7 @@
 #include <string.h>
 #include <sys/stat.h>
 #include <unistd.h>
+#include "cst_test_util.h"
 
 static int nfail;
 
@@ -33,11 +34,11 @@ static void put(const char *rel, const char *text) {
   for (char *q = p + strlen(g_root) + 1; *q; q++) {
     if (*q == '/') {
       *q = 0;
-      mkdir(p, 0755);
+      test_mkdir(p, 0755);
       *q = '/';
     }
   }
-  FILE *fp = fopen(p, "w");
+  FILE *fp = fopen(p, "wb");
   if (fp) {
     fputs(text, fp);
     fclose(fp);
@@ -70,8 +71,10 @@ static void run(const char *ws, const char *tool, const char *k1,
 }
 
 int main(void) {
-  snprintf(g_root, sizeof(g_root), "/tmp/cst-plugins-%d", (int)getpid());
-  mkdir(g_root, 0755);
+  if (test_tmpdir(g_root, sizeof(g_root), "cst-plugins") != 0) {
+    perror("tmpdir");
+    return 1;
+  }
   char ws[300], cfgdir[300];
   snprintf(ws, sizeof(ws), "%s/ws", g_root);
   snprintf(cfgdir, sizeof(cfgdir), "%s/cfg", g_root);
@@ -206,9 +209,7 @@ int main(void) {
   CHECK(!cst_tool_find("skill") && !cst_tool_find("echo_args"),
         "extensions unload");
 
-  char cmd[400];
-  snprintf(cmd, sizeof(cmd), "rm -rf %s", g_root);
-  if (system(cmd) != 0)
+  if (test_rmrf(g_root) != 0)
     fprintf(stderr, "note: cleanup failed\n");
   if (nfail) {
     fprintf(stderr, "test_plugins: %d failure(s)\n", nfail);