fix: improve shell compatibility and process cleanup

Preserve literal paths and arguments, use portable command options, validate startup configuration, and retain scripts when downloads fail.

Ensure task watchdogs and process groups are cleaned up across Bash versions, with compatibility and process lifecycle regression coverage.

shell/task-timeout.sh

@@ -36,7 +36,10 @@ run_task_command() {
     ) &
     timeout_watcher=$!
     set +m
-    trap '{
+    finish_task_timeout() {
+      local code="$1"
+      trap - EXIT
+      {
         # Bash can report a killed job between any two cleanup commands.
         # Keep redirection active until the watchdog is fully reaped.
         kill -KILL -- "-$timeout_watcher" || :
@@ -45,14 +48,19 @@ run_task_command() {
       if kill -TERM -- "-$timeout_worker" 2>/dev/null; then sleep 0.1; fi
       kill -KILL -- "-$timeout_worker" 2>/dev/null || :
       wait "$timeout_worker" 2>/dev/null || :
-      rm -rf -- "$timeout_dir"' EXIT
-    trap 'exit 130' INT
-    trap 'exit 143' TERM
-    trap 'exit 129' HUP
-    trap 'exit 131' QUIT
+      rm -rf -- "$timeout_dir"
+      exit "$code"
+    }
+    # Older Bash can omit a subshell's EXIT trap in sourced pipelines. Call
+    # cleanup explicitly on completion/signals, retaining EXIT as a fallback.
+    trap 'finish_task_timeout "$?"' EXIT
+    trap 'finish_task_timeout 130' INT
+    trap 'finish_task_timeout 143' TERM
+    trap 'finish_task_timeout 129' HUP
+    trap 'finish_task_timeout 131' QUIT
     # Do not clean up a timer whose process group has not initialized yet.
     while [[ ! -f "$timeout_dir/ready" ]]; do
-      kill -0 "$timeout_watcher" 2>/dev/null || exit 125
+      kill -0 "$timeout_watcher" 2>/dev/null || finish_task_timeout 125
       sleep 0.001
     done
     wait "$timeout_worker"
@@ -60,8 +68,8 @@ run_task_command() {
     if [[ -f "$timeout_dir/expired" ]]; then
       # Let the timer finish escalation even if the task leader exited first.
       wait "$timeout_watcher" 2>/dev/null || :
-      exit 124
+      finish_task_timeout 124
     fi
-    exit "$timeout_code"
+    finish_task_timeout "$timeout_code"
   )
 }