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/skills.c

@@ -13,10 +13,7 @@
 #include <string.h>
 #include <sys/stat.h>
 
-#ifndef _WIN32
-#include <dirent.h>
 #include <pthread.h>
-#endif
 
 #include "cst_tools.h"
 #include "cst_util.h"
@@ -166,14 +163,9 @@ static struct {
   char source[48];
 } g_roots[EXTRA_ROOTS];
 static int g_nroots;
-#ifndef _WIN32
 static pthread_mutex_t g_mu = PTHREAD_MUTEX_INITIALIZER;
 #define LOCK() pthread_mutex_lock(&g_mu)
 #define UNLOCK() pthread_mutex_unlock(&g_mu)
-#else
-#define LOCK() ((void)0)
-#define UNLOCK() ((void)0)
-#endif
 
 void cst_skills_add_root(const char *dir, const char *source) {
   if (!dir || !dir[0])
@@ -225,6 +217,7 @@ static char *slurp(const char *path, size_t max, size_t *len) {
   }
   if (!b.buf)
     cst_buf_puts(&b, "");
+  cst_strip_bom(b.buf, &b.len);
   if (len)
     *len = b.len;
   return cst_buf_steal(&b);
@@ -243,16 +236,15 @@ static int have(const cst_skill_t *v, int n, const char *name) {
 
 static int scan_root(const char *root, const char *source, cst_skill_t *out,
                      int n, int max) {
-#ifndef _WIN32
-  DIR *d = opendir(root);
+  os_dir_t *d = os_dir_open(root);
   if (!d)
     return n;
-  struct dirent *de;
-  while (n < max && (de = readdir(d)) != NULL) {
-    if (de->d_name[0] == '.')
+  const os_dirent_t *de;
+  while (n < max && (de = os_dir_next(d)) != NULL) {
+    if (de->name[0] == '.')
       continue;
     char dir[OS_PATH_MAX], md[OS_PATH_MAX + 16];
-    if (snprintf(dir, sizeof(dir), "%s/%s", root, de->d_name) >=
+    if (snprintf(dir, sizeof(dir), "%s/%s", root, de->name) >=
         (int)sizeof(dir))
       continue;
     snprintf(md, sizeof(md), "%s/SKILL.md", dir);
@@ -262,7 +254,7 @@ static int scan_root(const char *root, const char *source, cst_skill_t *out,
     cst_skill_t s;
     memset(&s, 0, sizeof(s));
     if (!cst_fm_get(text, "name", s.name, sizeof(s.name)) || !s.name[0])
-      snprintf(s.name, sizeof(s.name), "%.63s", de->d_name);
+      snprintf(s.name, sizeof(s.name), "%.63s", de->name);
     cst_fm_get(text, "description", s.desc, sizeof(s.desc));
     if (!s.desc[0]) {
       /* first prose line of the body */
@@ -282,13 +274,7 @@ static int scan_root(const char *root, const char *source, cst_skill_t *out,
     if (!have(out, n, s.name))
       out[n++] = s;
   }
-  closedir(d);
-#else
-  (void)root;
-  (void)source;
-  (void)out;
-  (void)max;
-#endif
+  os_dir_close(d);
   return n;
 }
 
@@ -371,25 +357,24 @@ static int find_skill(const char *ws, const char *name, cst_skill_t *out) {
 
 static void list_files(const char *dir, const char *rel, int depth,
                        cst_buf_t *b, int *count) {
-#ifndef _WIN32
   char full[OS_PATH_MAX];
   if (snprintf(full, sizeof(full), "%s%s%s", dir, rel[0] ? "/" : "", rel) >=
       (int)sizeof(full))
     return;
-  DIR *d = opendir(full);
+  os_dir_t *d = os_dir_open(full);
   if (!d)
     return;
   char *names[256];
   int nn = 0;
-  struct dirent *de;
-  while (nn < 256 && (de = readdir(d)) != NULL) {
-    if (de->d_name[0] == '.')
+  const os_dirent_t *de;
+  while (nn < 256 && (de = os_dir_next(d)) != NULL) {
+    if (de->name[0] == '.')
       continue;
-    names[nn] = strdup(de->d_name);
+    names[nn] = strdup(de->name);
     if (names[nn])
       nn++;
   }
-  closedir(d);
+  os_dir_close(d);
   for (int i = 1; i < nn; i++) {
     for (int j = i; j > 0 && strcmp(names[j - 1], names[j]) > 0; j--) {
       char *tmp = names[j];
@@ -416,13 +401,6 @@ static void list_files(const char *dir, const char *rel, int depth,
   }
   for (int i = 0; i < nn; i++)
     free(names[i]);
-#else
-  (void)dir;
-  (void)rel;
-  (void)depth;
-  (void)b;
-  (void)count;
-#endif
 }
 
 static char *errf(int *code, const char *fmt, const char *a, const char *b) {