Give the sandbox a UTF-8 locale

A sandbox image ships without a locale, leaving LC_CTYPE at POSIX. Every
multibyte glyph then degrades to a placeholder, so Nerd Font icons in the
editor render as underscores and bash printf emits \uXXXX escapes literally.

The locale is probed for usability rather than matched by name, because
locale -a spells C.UTF-8 as C.utf8 on glibc and a name match would silently
find nothing. LANG alone is set, leaving individual categories overridable.

tasks/ai/sbx

@@ -1055,6 +1055,39 @@ EOF
     '
 }
 
+# A sandbox image ships without a locale, which leaves LC_CTYPE at POSIX. Every
+# multibyte glyph then degrades to a placeholder: Nerd Font icons in the editor
+# render as underscores, and bash printf emits \uXXXX escapes literally. LANG
+# alone is enough, and leaves a user free to override individual categories.
+install_sandbox_locale() {
+    # shellcheck disable=SC2016
+    sbx exec "$SANDBOX_NAME" bash -c '
+        persistent=/etc/sandbox-persistent.sh
+        marker="# BEGIN ai-sbx locale"
+
+        if grep -Fq "$marker" "$persistent" 2>/dev/null; then
+            exit 0
+        fi
+
+        # locale -a spells these inconsistently across distributions, so probe
+        # each one for usability rather than matching its name.
+        for candidate in C.UTF-8 en_US.UTF-8; do
+            if LC_ALL="$candidate" locale >/dev/null 2>&1; then
+                chosen="$candidate"
+                break
+            fi
+        done
+
+        [[ -n "${chosen:-}" ]] || exit 0
+
+        cat >>"$persistent" <<EOF
+# BEGIN ai-sbx locale
+export LANG=$chosen
+# END ai-sbx locale
+EOF
+    ' </dev/null >/dev/null 2>&1 || true
+}
+
 validate_launch_mode() {
     case "$1" in
         agent | tmux) ;;
@@ -1427,6 +1460,8 @@ run_command() {
     # runs every time rather than only at setup.
     install_sandbox_mise
 
+    install_sandbox_locale
+
     if [[ "$launch" == tmux ]]; then
         install_sandbox_workspace