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_msys2.sh

@@ -0,0 +1,58 @@
+#!/usr/bin/env bash
+# Native Windows build from an MSYS2 UCRT64 shell (https://www.msys2.org/).
+#   scripts/bootstrap_msys2.sh [repo-dir]
+# Installs the UCRT64 toolchain with pacman, builds the same pinned static
+# deps as the Linux cross build (curl with Schannel, ncurses with the Windows
+# console patches, busybox-w32 Unicode), then a static cst-agent.exe with the
+# UTF-8 manifest, runs the tests and packages dist/windows-x86_64/.
+# CST_SKIP_TESTS=1 skips ctest.
+set -euo pipefail
+
+if [[ "${MSYSTEM:-}" != UCRT64 ]]; then
+  echo "run this from the MSYS2 UCRT64 shell: the UTF-8 manifest needs the" \
+    "UCRT runtime (MINGW64 links msvcrt)" >&2
+  exit 1
+fi
+
+REPO_URL="${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="$(pwd)"
+  else
+    DIR="CST-agent"
+  fi
+fi
+
+pacman -S --needed --noconfirm \
+  mingw-w64-ucrt-x86_64-gcc mingw-w64-ucrt-x86_64-cmake \
+  mingw-w64-ucrt-x86_64-ninja make git patch tar perl zip diffutils
+
+if [[ ! -f "$DIR/CMakeLists.txt" ]]; then
+  echo "==> git clone $REPO_URL -> $DIR"
+  git clone "$REPO_URL" "$DIR"
+fi
+cd "$DIR"
+
+# third-party script: unprefixed native tools
+export MINGW_HOST= BUILD_CC=gcc
+[[ -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
+if [[ ! -f third_party/busybox/busybox.exe ]]; then
+  scripts/build_third_party_mingw.sh busybox-w32 || scripts/fetch_busybox_w32.sh
+fi
+
+echo "==> cmake (MSYS2 UCRT64, static, TUI)"
+cmake -S . -B build-msys2 -G Ninja -DCMAKE_BUILD_TYPE=Release \
+  -DCMAKE_PREFIX_PATH="$PWD/third_party/prefix-mingw" \
+  -DCST_WIN_STATIC=ON -DCST_BUILD_TUI=ON
+cmake --build build-msys2
+
+if [[ -z "${CST_SKIP_TESTS:-}" ]]; then
+  echo "==> ctest"
+  cp -f third_party/busybox/busybox.exe build-msys2/
+  ctest --test-dir build-msys2 --output-on-failure
+fi
+
+echo "==> package"
+CST_WIN_BUILD=build-msys2 scripts/package_windows.sh