Sandbox: Wurzel schreibgeschuetzt, Filter-Deskriptor geschlossen (#654)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/ClawdDotNet.Core/Sandbox/BwrapSandbox.cs

@@ -170,7 +170,9 @@ public sealed class BwrapSandbox : ISandboxRunner
         foreach (var path in request.ReadOnlyPaths)
             args.AddRange(["--ro-bind", Path.GetFullPath(path), Path.GetFullPath(path)]);
 
-        args.AddRange(["--chdir", SandboxPaths.Work]);
+        // The root bwrap builds is a tmpfs of its own — writable unless it is remounted. Only
+        // /work and /tmp, each a mount of its own, stay writable.
+        args.AddRange(["--remount-ro", "/", "--chdir", SandboxPaths.Work]);
 
         var environment = new Dictionary<string, string>(StringComparer.Ordinal)
         {
@@ -186,9 +188,10 @@ public sealed class BwrapSandbox : ISandboxRunner
 
         args.AddRange(["--seccomp", "3", "--"]);
 
-        // Inner: the process limit, set inside the sandbox's own user namespace. bash calls it
-        // -u, dash (Debian's and Ubuntu's sh) -p; if neither takes, nothing runs.
-        args.AddRange([Shell, "-c", """ulimit -u "$1" 2>/dev/null || ulimit -p "$1" || exit 126; shift; exec "$@" """, "sh",
+        // Inner: fd 3 — the filter bwrap read — is closed; bwrap hands it on otherwise. Then
+        // the process limit, set inside the sandbox's own user namespace. bash calls it -u,
+        // dash (Debian's and Ubuntu's sh) -p; if neither takes, nothing runs.
+        args.AddRange([Shell, "-c", """exec 3<&-; ulimit -u "$1" 2>/dev/null || ulimit -p "$1" || exit 126; shift; exec "$@" """, "sh",
             limits.MaxProcesses.ToString(CultureInfo.InvariantCulture), request.Executable]);
         args.AddRange(request.Arguments);
         return args;