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_tools_fs.c

@@ -12,6 +12,8 @@
 #include <sys/stat.h>
 #include <unistd.h>
 
+#include "cst_test_util.h"
+
 static int nfail;
 
 #define EXPECT(cond, msg)                                                      \
@@ -98,17 +100,19 @@ static int run(cst_tool_ctx_t *ctx, ctl_tool_call_t c, cst_tool_result_t *r) {
 static void reset(cst_tool_result_t *r) { cst_tool_result_free(r); }
 
 int main(void) {
-  snprintf(g_ws, sizeof(g_ws), "/tmp/cst-tools-test-%d", (int)getpid());
-  mkdir(g_ws, 0755);
+  if (test_tmpdir(g_ws, sizeof(g_ws), "cst-tools-test") != 0) {
+    perror("tmpdir");
+    return 1;
+  }
   char p[512];
   snprintf(p, sizeof(p), "%s/src", g_ws);
-  mkdir(p, 0755);
+  test_mkdir(p, 0755);
   snprintf(p, sizeof(p), "%s/src/deep", g_ws);
-  mkdir(p, 0755);
+  test_mkdir(p, 0755);
   snprintf(p, sizeof(p), "%s/node_modules", g_ws);
-  mkdir(p, 0755);
+  test_mkdir(p, 0755);
   snprintf(p, sizeof(p), "%s/build", g_ws);
-  mkdir(p, 0755);
+  test_mkdir(p, 0755);
   wfile("src/main.c", "#include <stdio.h>\nint main(void) {\n  int x = 42;\n"
                       "  printf(\"%d\\n\", x);\n  return 0;\n}\n");
   wfile("src/deep/util.c", "int add(int a, int b) { return a + b; }\n"
@@ -119,7 +123,7 @@ int main(void) {
   wfile(".gitignore", "build/\n*.log\n");
   wfile("run.log", "int add in log\n");
   {
-    FILE *fp = fopen(strcat(strcpy(p, g_ws), "/lines.txt"), "w");
+    FILE *fp = fopen(strcat(strcpy(p, g_ws), "/lines.txt"), "wb");
     for (int i = 1; i <= 100; i++)
       fprintf(fp, "line %d\n", i);
     fclose(fp);
@@ -410,9 +414,7 @@ int main(void) {
            "patch targets");
   }
 
-  char cmd[400];
-  snprintf(cmd, sizeof(cmd), "rm -rf %s", g_ws);
-  if (system(cmd) != 0)
+  if (test_rmrf(g_ws) != 0)
     fprintf(stderr, "note: cleanup failed\n");
   if (nfail) {
     fprintf(stderr, "test_tools_fs: %d failure(s)\n", nfail);