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_mcp.c

@@ -14,7 +14,10 @@
 #include <string.h>
 #include <unistd.h>
 
+#include "cst_test_util.h"
+
 static int nfail;
+static char g_tmp[OS_PATH_MAX];
 
 #define CHECK(cond, msg)                                                       \
   do {                                                                         \
@@ -159,7 +162,7 @@ static int call(const char *tool, const char *k, const char *v,
   }
   cst_tool_ctx_t ctx;
   memset(&ctx, 0, sizeof(ctx));
-  ctx.workspace_root = "/tmp";
+  ctx.workspace_root = g_tmp;
   memset(r, 0, sizeof(*r));
   return cst_tool_run(&ctx, &c, r);
 }
@@ -181,6 +184,8 @@ int main(int argc, char **argv) {
     fprintf(stderr, "cannot resolve %s\n", argv[0]);
     return 1;
   }
+  if (os_temp_dir(g_tmp, sizeof(g_tmp)) != 0)
+    snprintf(g_tmp, sizeof(g_tmp), ".");
   setenv("CST_TEST_SECRET", "s3cret", 1);
   char cfg[OS_PATH_MAX + 128];
   snprintf(cfg, sizeof(cfg),
@@ -206,7 +211,7 @@ int main(int argc, char **argv) {
   cst_mcp_counts(&ready, &total);
   CHECK(ready == 0 && total == 3, "counts before start (disabled excluded)");
 
-  cst_mcp_start_all("/tmp");
+  cst_mcp_start_all(g_tmp);
   cst_mcp_wait_ready(10000);
 
   cst_mcp_info_t mi[CST_MCP_MAX];
@@ -281,7 +286,7 @@ int main(int argc, char **argv) {
 
   /* a live server stops cleanly too */
   CHECK(cst_mcp_add_config_json("fake2", cfg, NULL, "user") == 0, "re-add");
-  cst_mcp_start_all("/tmp");
+  cst_mcp_start_all(g_tmp);
   cst_mcp_wait_ready(10000);
   CHECK(cst_tool_find("mcp__fake2__echo"), "second server ready");
   t0 = os_now_ms();