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

src/creds.c

@@ -12,6 +12,7 @@
 
 #ifdef _WIN32
 #include <io.h>
+#include <windows.h>
 #else
 #include <fcntl.h>
 #include <termios.h>
@@ -51,7 +52,7 @@ int cst_creds_load(const char *provider, char *out, size_t out_sz) {
   char path[OS_PATH_MAX];
   if (credentials_path(path, sizeof(path)) != 0)
     return -1;
-  FILE *fp = fopen(path, "r");
+  FILE *fp = fopen(path, "rb");
   if (!fp)
     return -1;
   char want[64];
@@ -104,7 +105,9 @@ int cst_creds_save(const char *provider, const char *key) {
   size_t wl = strlen(want);
 
 #ifdef _WIN32
-  FILE *out = fopen(tmp, "w");
+  FILE *out = fopen(tmp, "wb");
+  if (out)
+    (void)os_chmod_private(tmp); /* label before the key is written */
 #else
   /* never exists world-readable, even briefly */
   int fd = open(tmp, O_WRONLY | O_CREAT | O_TRUNC, 0600);
@@ -114,7 +117,7 @@ int cst_creds_save(const char *provider, const char *key) {
 #endif
   if (!out)
     return -1;
-  FILE *in = fopen(path, "r");
+  FILE *in = fopen(path, "rb");
   if (in) {
     char line[1024];
     while (fgets(line, sizeof(line), in)) {
@@ -128,10 +131,7 @@ int cst_creds_save(const char *provider, const char *key) {
     remove(tmp);
     return -1;
   }
-#ifdef _WIN32
-  remove(path);
-#endif
-  if (rename(tmp, path) != 0) {
+  if (os_rename_replace(tmp, path) != 0) {
     remove(tmp);
     return -1;
   }
@@ -167,7 +167,13 @@ int cst_creds_prompt_console(const char *provider, char *out, size_t out_sz) {
   fprintf(stderr, "API key for %s (input hidden, saved to the config dir): ",
           provider ? provider : "?");
   fflush(stderr);
-#ifndef _WIN32
+#ifdef _WIN32
+  HANDLE hin = GetStdHandle(STD_INPUT_HANDLE);
+  DWORD old_mode = 0;
+  int tty = GetConsoleMode(hin, &old_mode) != 0;
+  if (tty)
+    SetConsoleMode(hin, old_mode & ~(DWORD)ENABLE_ECHO_INPUT);
+#else
   struct termios old, quiet;
   int tty = isatty(STDIN_FILENO) && tcgetattr(STDIN_FILENO, &old) == 0;
   if (tty) {
@@ -177,12 +183,14 @@ int cst_creds_prompt_console(const char *provider, char *out, size_t out_sz) {
   }
 #endif
   char *got = fgets(out, (int)out_sz, stdin);
-#ifndef _WIN32
   if (tty) {
+#ifdef _WIN32
+    SetConsoleMode(hin, old_mode);
+#else
     tcsetattr(STDIN_FILENO, TCSAFLUSH, &old);
+#endif
     fputc('\n', stderr);
   }
-#endif
   if (!got) {
     out[0] = 0;
     return -1;