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

include/os_compat.h

@@ -22,6 +22,28 @@ int os_realpath(const char *path, char *out, size_t out_sz);
 /* True if path is under root (after realpath/norm). Trailing slash rules apply. */
 int os_path_under(const char *root, const char *path);
 
+/* --- directory listing / file type --- */
+typedef struct os_dir os_dir_t;
+typedef struct {
+  const char *name; /* UTF-8; valid until the next os_dir_next */
+  int is_dir;
+  int is_link; /* symlink, or a junction on Windows (not followed) */
+} os_dirent_t;
+os_dir_t *os_dir_open(const char *path);
+/* Next entry, skipping "." and ".."; NULL at the end. */
+const os_dirent_t *os_dir_next(os_dir_t *d);
+void os_dir_close(os_dir_t *d);
+
+typedef struct {
+  int is_dir, is_reg, is_link;
+  long long size;
+  double mtime; /* seconds since the epoch */
+} os_stat_t;
+/* Like lstat(2): a link is reported as a link, not followed. */
+int os_lstat(const char *path, os_stat_t *st);
+/* Replace `to` if it exists (atomic where the OS allows). */
+int os_rename_replace(const char *from, const char *to);
+
 /* --- dirs / config --- */
 int os_mkdir(const char *path, int mode);
 int os_mkdir_p(const char *path, int mode);
@@ -67,6 +89,28 @@ int os_run_shell_ex(const char *cwd, const char *cmd, size_t max_out,
                     os_run_result_t *r, os_child_hook_fn child_hook,
                     void *ud);
 
+#ifdef _WIN32
+/* Windows: run under `token` (a primary token, e.g. the Low-IL sandbox
+ * token; NULL = ours) with env_add "KEY=VALUE" entries layered on the
+ * inherited environment. Every child runs in a kill-on-close Job Object, so
+ * timeouts and cancels end the whole process tree. */
+int os_run_capture_token(const char *cwd, const char *const argv[],
+                         size_t max_out, os_run_result_t *r, void *token,
+                         const char *const env_add[]);
+/* os_run_shell_c under token, with env_add. */
+int os_run_shell_token(const char *cwd, const char *cmd, size_t max_out,
+                       os_run_result_t *r, void *token,
+                       const char *const env_add[]);
+/* "Windows 10.0.26100 x86_64" (the real version, not the compat shim's). */
+void os_windows_version(char *out, size_t n);
+/* Sets the mandatory-label SACL, e.g. "S:(ML;OICI;NW;;;LW)"; 0 / -1. */
+int os_win_set_label(const char *path, const char *sddl);
+/* The explicit (not inherited) label as SDDL, "S:" when there is none. */
+int os_win_get_label(const char *path, char *out, size_t n);
+/* CF_UNICODE_TEXT clipboard (the console may not do OSC 52). */
+int os_clipboard_set(const char *utf8, size_t n);
+#endif
+
 /* Child processes get stdin=/dev/null, their own process group, and are
  * killed after the timeout (CST_SHELL_TIMEOUT seconds, default 120) or when
  * *flag becomes non-zero. Exit code 124 = timeout, 130 = cancelled. */
@@ -80,7 +124,7 @@ int os_get_run_timeout(void);
  * the inherited environment (NULL-terminated, may be NULL). */
 typedef struct {
 #ifdef _WIN32
-  void *proc, *in_w, *out_r;
+  void *proc, *in_w, *out_r, *job;
 #else
   int pid, in_fd, out_fd;
 #endif
@@ -106,10 +150,30 @@ void os_run_parallel(os_task_fn fn, void *const args[], int n);
 /* Monotonic milliseconds (arbitrary epoch). */
 double os_now_ms(void);
 
+/* --- text encoding ---
+ * cst-agent is UTF-8 throughout. On Windows the manifest makes UTF-8 the
+ * process code page; os_console_init() also switches the console to UTF-8
+ * (restored at exit), sets a UTF-8 C locale and asks Python children for
+ * UTF-8. Returns -1 when the process code page is not UTF-8 (pre-1903
+ * Windows); no-op returning 0 elsewhere. */
+int os_console_init(void);
+/* Child output that is not valid UTF-8, decoded from the console/OEM/ANSI
+ * code page: malloc'd UTF-8, or NULL when s is already UTF-8 (or not on
+ * Windows). */
+char *os_output_to_utf8(const char *s, size_t n, size_t *out_len);
+#ifdef _WIN32
+#include <wchar.h>
+/* malloc'd conversions; NULL on invalid input or allocation failure */
+wchar_t *os_utf8_to_wide(const char *s);
+char *os_wide_to_utf8(const wchar_t *w);
+#endif
+
 /* --- misc --- */
 void os_localtime(const time_t *t, struct tm *out);
 int os_getpid(void);
-int os_chmod_private(const char *path); /* best-effort 0600; no-op/ok on Win */
+/* 0600; on Windows a Medium no-read-up label, so Low-IL children (the
+ * shell sandbox) cannot read the file. */
+int os_chmod_private(const char *path);
 
 /* --- UTF-8 --------------------------------------------------------------
  * Encode one code point as UTF-8, independent of the C locale.