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.

tests/launch-mode.test.sh

@@ -47,8 +47,12 @@ dispatch() {
     cat "$SBX_LOG"
 }
 
-[[ "$DEFAULT_LAUNCH" == agent ]] ||
-    fail "AI_SBX_LAUNCH unset should default to agent, got: $DEFAULT_LAUNCH"
+(
+    unset AI_SBX_LAUNCH
+    # shellcheck source=tasks/ai/sbx
+    source "$TASK"
+    [[ "$DEFAULT_LAUNCH" == agent ]]
+) || fail "AI_SBX_LAUNCH unset should default to agent"
 
 (
     AI_SBX_LAUNCH=tmux
@@ -87,6 +91,13 @@ DEFAULT_LAUNCH=agent
 [[ "$(dispatch -- --launch tmux)" == *"run ai-test -- --launch tmux"* ]] ||
     fail "--launch after -- belongs to the agent, not to the task"
 
+DEFAULT_LAUNCH=agent
+locale_dispatch="$(dispatch)"
+[[ "$locale_dispatch" == *"BEGIN ai-sbx locale"* ]] ||
+    fail "run should install a UTF-8 locale, or Nerd Font glyphs render as placeholders: $locale_dispatch"
+[[ "$locale_dispatch" == *"LC_ALL=\"\$candidate\" locale"* ]] ||
+    fail "the locale must be probed for usability, not matched by name against locale -a"
+
 if (validate_launch_mode bogus) 2>/dev/null; then
     fail "an unknown launch mode was accepted"
 fi