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

scripts/bootstrap_windows.sh

@@ -1,11 +1,13 @@
 #!/usr/bin/env bash
-# One-shot on a Linux host: cross-build Windows x86_64 release (MinGW).
-# Output: dist/windows-x86_64/  — copy that folder to a Windows machine.
+# Linux host: cross-build the Windows x86_64 release (mingw-w64, UCRT, static).
+#   scripts/bootstrap_windows.sh [repo-dir]
+# With Nix the toolchain comes from shell-win.nix; otherwise x86_64-w64-mingw32-gcc
+# (UCRT flavour, with winpthreads), cmake, ninja, git and perl must be installed.
+# Output: dist/windows-x86_64/ and dist/cst-agent-VERSION-windows-x86_64.zip
 set -euo pipefail
 
 REPO_URL="${CST_AGENT_REPO:-https://git.whcself.xyz/WHC/CST-agent.git}"
 DIR="${1:-}"
-
 if [[ -z "$DIR" ]]; then
   if [[ -f CMakeLists.txt && -d src ]]; then
     DIR="$(pwd)"
@@ -13,36 +15,33 @@ if [[ -z "$DIR" ]]; then
     DIR="CST-agent"
   fi
 fi
-
 if [[ ! -f "$DIR/CMakeLists.txt" ]]; then
   echo "==> git clone $REPO_URL -> $DIR"
   git clone "$REPO_URL" "$DIR"
 fi
 cd "$DIR"
 
-need() { command -v "$1" >/dev/null || { echo "missing $1 (install mingw-w64 gcc cmake git)" >&2; exit 1; }; }
-need git; need cmake; need x86_64-w64-mingw32-gcc
+if ! command -v x86_64-w64-mingw32-gcc >/dev/null && [[ -z "${IN_NIX_SHELL:-}" ]] &&
+  command -v nix-shell >/dev/null; then
+  exec nix-shell shell-win.nix --run "$(printf '%q ' "$0" "$PWD")"
+fi
 
-echo "==> third-party (curl Schannel + PDCurses + busybox-w32)"
-chmod +x scripts/*.sh 2>/dev/null || true
-./scripts/build_third_party_mingw.sh all
+need() { command -v "$1" >/dev/null || { echo "missing $1 (mingw-w64 UCRT gcc, cmake, ninja, git, perl)" >&2; exit 1; }; }
+need git; need cmake; need ninja; need perl; need x86_64-w64-mingw32-gcc
+if ! printf '#include <stdio.h>\n' | x86_64-w64-mingw32-gcc -dM -E - | grep -qw _UCRT; then
+  echo "x86_64-w64-mingw32-gcc targets msvcrt; the UTF-8 manifest needs a UCRT toolchain" >&2
+  exit 1
+fi
 
-echo "==> cmake Windows TUI"
-cmake -B build-win-tui \
+[[ -f third_party/prefix-mingw/lib/libcurl.a ]] || scripts/build_third_party_mingw.sh curl
+[[ -f third_party/prefix-mingw/lib/libncursesw.a ]] || scripts/build_third_party_mingw.sh ncurses
+[[ -f third_party/busybox/busybox.exe ]] || scripts/build_third_party_mingw.sh busybox-w32
+
+echo "==> cmake (Windows, TUI)"
+cmake -S . -B build-win-tui -G Ninja \
   -DCMAKE_TOOLCHAIN_FILE=cmake/mingw-w64-x86_64.cmake \
-  -DCST_BUILD_TUI=ON \
-  -DCMAKE_PREFIX_PATH="$PWD/third_party/prefix-mingw" \
-  -DPDCurses_ROOT="$PWD/third_party/prefix-mingw"
-cmake --build build-win-tui -j"$(nproc 2>/dev/null || echo 4)"
+  -DCMAKE_BUILD_TYPE=Release -DCST_BUILD_TUI=ON
+cmake --build build-win-tui
 
 echo "==> package"
-./scripts/package_windows.sh
-
-echo
-echo "OK: dist/windows-x86_64/"
-echo "Copy that folder to Windows, then:"
-echo "  set OPENAI_API_KEY=sk-..."
-echo "  set OPENAI_BASE_URL=https://api.openai.com/v1"
-echo "  set OPENAI_MODEL=gpt-4.1-mini"
-echo "  cst-agent.exe --workspace ."
-echo "See docs/env_openai.md and docs/README-windows.md"
+scripts/package_windows.sh