fix: Codex watchdog freezes the command before reaping its children

The bash-native watchdog (stock macOS, no timeout binary) ran pkill -P on
the command's children and then killed the command. Between the two, the
command continued past its killed child and printed output after its
deadline. Stop it first. The test's minimal PATH also lacked pkill, which
the watchdog uses to reap children (stock macOS ships it), so the orphaned
child held stdout open and the capture waited out its sleep.

bin/gstack-codex-probe

@@ -418,6 +418,9 @@ _gstack_codex_timeout_wrapper() {
         sleep 1
         _GSTACK_CODEX_WAITED=$((_GSTACK_CODEX_WAITED + 1))
       done
+      # Freeze the command first: killing its children while it runs lets it
+      # continue past the killed child and print output after its deadline.
+      kill -STOP "$_cmd_pid" 2>/dev/null
       pkill -KILL -P "$_cmd_pid" 2>/dev/null
       kill -KILL "$_cmd_pid" 2>/dev/null
       exit 124

test/codex-hardening.test.ts

@@ -357,7 +357,10 @@ describe('gstack-codex-probe: timeout wrapper + namespace hygiene', () => {
     // tool gate killed the whole call and the partial output was lost.
     const dir = fs.mkdtempSync(path.join(os.tmpdir(), 'gstack-watchdog-stubborn-'));
     try {
-      for (const tool of native ? ['bash', 'sleep', 'cat'] : []) {
+      // pkill is part of the watchdog's contract (it reaps the provider's
+      // children; stock macOS ships /usr/bin/pkill). Without it the orphaned
+      // `sleep 30` keeps stdout open and the capture waits out the sleep.
+      for (const tool of native ? ['bash', 'sleep', 'cat', 'pkill'] : []) {
         const resolved = spawnSync('bash', ['-c', `command -v ${tool}`], { timeout: 5000 });
         fs.symlinkSync(resolved.stdout.toString().trim(), path.join(dir, tool));
       }