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/cst_sandbox.h

@@ -12,6 +12,10 @@
  *           against mistakes, not hostile code (the path is checked before
  *           the kernel reads it again).
  *   macOS   sandbox_init (Seatbelt profile)
+ *   Windows Low integrity level: a restricted token (no admin privileges,
+ *           Low IL) can only write objects labeled Low, so the workspace and
+ *           a temp dir get that label; the credentials file is labeled
+ *           no-read-up. The network stays open (callers ask first).
  *   other   none: callers fall back to rule-based approval
  */
 
@@ -27,15 +31,16 @@ typedef enum {
   CST_SANDBOX_NONE = 0,
   CST_SANDBOX_LANDLOCK,
   CST_SANDBOX_SEATBELT,
-  CST_SANDBOX_SECCOMP
+  CST_SANDBOX_SECCOMP,
+  CST_SANDBOX_LOWIL
 } cst_sandbox_kind_t;
 
 /* What this process can use; cached. CST_SANDBOX=off|0|none forces NONE,
- * CST_SANDBOX=landlock|seccomp allows only that kind. */
+ * CST_SANDBOX=landlock|seccomp|lowil allows only that kind. */
 int cst_sandbox_probe(void);
 /* Drops the cache and probes again (tests that change CST_SANDBOX). */
 int cst_sandbox_reprobe(void);
-/* "landlock" | "seatbelt" | "seccomp" | "none" */
+/* "landlock" | "seatbelt" | "seccomp" | "lowil" | "none" */
 const char *cst_sandbox_name(int kind);
 /* Landlock ABI version (0 when unavailable); ≥4 also blocks TCP. */
 int cst_sandbox_landlock_abi(void);
@@ -45,6 +50,9 @@ typedef struct {
   int ruleset_fd; /* Landlock, built in the parent */
   char *profile;  /* Seatbelt */
   void *sc;       /* seccomp: filter program and supervisor thread */
+  void *token;    /* lowil: primary token for CreateProcessAsUserW */
+  char *tmpdir;   /* lowil: Low-labeled TEMP for the child */
+  char *note;     /* lowil: shown once when a directory was just labeled */
 } cst_sandbox_plan_t;
 
 /* Parent side: build the policy. Returns 0, or -1 (plan->kind = NONE). */
@@ -64,6 +72,14 @@ int cst_sandbox_run_shell(const char *workspace, const char *cmd,
  * way (EACCES / EPERM / EROFS / blocked network). */
 int cst_sandbox_blocked(const char *out, size_t n, int exit_code);
 
+/* lowil: directories this user's cst-agent labeled Low (recorded in the
+ * config dir), one per line; malloc'd, NULL when there are none. */
+char *cst_sandbox_labels(void);
+/* Gives every recorded directory its old label back. Returns how many were
+ * restored (-1 when some failed; those stay recorded); *report gets a
+ * malloc'd summary. */
+int cst_sandbox_reset_labels(char **report);
+
 #ifdef __cplusplus
 }
 #endif