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/plugins.c

@@ -11,10 +11,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include <sys/stat.h>
-#ifndef _WIN32
-#include <dirent.h>
 #include <unistd.h>
-#endif
 
 #include "cJSON.h"
 #include "cst_mcp.h"
@@ -60,6 +57,7 @@ static char *slurp(const char *path, size_t max) {
   fclose(fp);
   if (!b.buf)
     cst_buf_puts(&b, "");
+  cst_strip_bom(b.buf, &b.len);
   return cst_buf_steal(&b);
 }
 
@@ -101,14 +99,6 @@ static int plugin_run(const cst_tool_ctx_t *ctx, const ctl_tool_call_t *c,
                       cst_tool_result_t *r, void *ud) {
   const ptool_t *pt = ud;
   r->exit_code = 1;
-#ifdef _WIN32
-  (void)ctx;
-  (void)c;
-  (void)pt;
-  r->out = strdup("error: plugin tools are not supported on Windows yet");
-  r->out_len = r->out ? strlen(r->out) : 0;
-  return 0;
-#else
   const cst_tool_def_t *def = cst_tool_find(pt->tool);
   char *args = cst_tool_args_json(def, c);
   char tmp[OS_PATH_MAX];
@@ -180,7 +170,6 @@ static int plugin_run(const cst_tool_ctx_t *ctx, const ctl_tool_call_t *c,
   r->out_len = out.len;
   r->out = cst_buf_steal(&out);
   return 0;
-#endif
 }
 
 /* argv array -> quoted command line; relative entries found in the plugin
@@ -339,21 +328,20 @@ static void load_plugin(const char *dir, const char *dirname,
 
 static int scan(const char *root, const char *source) {
   int n = 0;
-#ifndef _WIN32
-  DIR *d = opendir(root);
+  os_dir_t *d = os_dir_open(root);
   if (!d)
     return 0;
   char *names[CST_PLUGINS_MAX * 2];
   int nn = 0;
-  struct dirent *de;
-  while (nn < CST_PLUGINS_MAX * 2 && (de = readdir(d)) != NULL) {
-    if (de->d_name[0] == '.')
+  const os_dirent_t *de;
+  while (nn < CST_PLUGINS_MAX * 2 && (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 *t = names[j];
@@ -371,32 +359,24 @@ static int scan(const char *root, const char *source) {
     }
     free(names[i]);
   }
-#else
-  (void)root;
-  (void)source;
-#endif
   return n;
 }
 
 static int count_plugin_dirs(const char *root) {
   int n = 0;
-#ifndef _WIN32
-  DIR *d = opendir(root);
+  os_dir_t *d = os_dir_open(root);
   if (!d)
     return 0;
-  struct dirent *de;
-  while ((de = readdir(d)) != NULL) {
+  const os_dirent_t *de;
+  while ((de = os_dir_next(d)) != NULL) {
     char pj[OS_PATH_MAX];
-    if (de->d_name[0] != '.' &&
-        snprintf(pj, sizeof(pj), "%s/%s/plugin.json", root, de->d_name) <
+    if (de->name[0] != '.' &&
+        snprintf(pj, sizeof(pj), "%s/%s/plugin.json", root, de->name) <
             (int)sizeof(pj) &&
         is_file(pj))
       n++;
   }
-  closedir(d);
-#else
-  (void)root;
-#endif
+  os_dir_close(d);
   return n;
 }