feat: Codex非同期実行システムの改良版実装

- codex-async-notify.sh: 並行制御・重複回避・ログ管理機能追加
  - CODEX_MAX_CONCURRENT: 最大同時実行数制御
  - CODEX_DEDUP: 同一タスクの重複起動防止
  - 古いログの自動クリーンアップ機能
  - より堅牢なプロセス検出（pgrep/psフォールバック）

- codex-keep-two.sh: プロセスカウント改善
  - pgrep使用で正確なプロセス検出
  - 日本語メッセージでの状態表示

- AGENT.md: Codex Async Workflow ドキュメント更新

🤖 Generated with [Claude Code](https://claude.ai/code)

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

tools/codex-keep-two.sh

@@ -0,0 +1,55 @@
+#!/bin/bash
+# Keep two codex exec jobs running by topping up when fewer are active.
+# Usage:
+#   tools/codex-keep-two.sh <tmux_session> "Task A" "Task B" ["Task C" ...]
+# Notes:
+#   - Detects only real `codex exec` processes via ps/awk (avoids self-matches).
+#   - Starts tasks in order, cycling if more top-ups are needed.
+
+set -euo pipefail
+
+if [ $# -lt 2 ]; then
+  echo "Usage: $0 <tmux_session> \"Task A\" \"Task B\" [\"Task C\" ...]" >&2
+  exit 1
+fi
+
+SESSION="$1"; shift
+TASKS=("$@")
+if [ ${#TASKS[@]} -eq 0 ]; then
+  echo "Provide at least one task string." >&2
+  exit 1
+fi
+
+CODEX_PROC_PATTERN=${CODEX_PROC_PATTERN:-'codex .* exec'}
+
+list_running() {
+  if command -v pgrep >/dev/null 2>&1; then
+    pgrep -af -- "$CODEX_PROC_PATTERN" | grep -v 'pgrep' || true
+  else
+    ps -eo pid=,args= | grep -E -- "$CODEX_PROC_PATTERN" | grep -v grep || true
+  fi
+}
+
+count_running() {
+  list_running | wc -l | tr -d ' '\
+  || echo 0
+}
+
+RUNNING=$(count_running)
+echo "[keep-two] 実際のcodexプロセス数: ${RUNNING}"
+NEED=$((2 - RUNNING))
+if [ $NEED -le 0 ]; then
+  echo "[keep-two] already running: $RUNNING (>=2)."
+  exit 0
+fi
+
+echo "[keep-two] running=$RUNNING, starting $NEED top-up task(s)."
+
+idx=0
+for ((i=0; i<NEED; i++)); do
+  TASK="${TASKS[$idx]}"; idx=$(((idx+1) % ${#TASKS[@]}))
+  echo "[keep-two] start: $TASK"
+  CODEX_ASYNC_DETACH=1 ./tools/codex-async-notify.sh "$TASK" "$SESSION" >/dev/null 2>&1 || true
+done
+
+echo "[keep-two] now running: $(count_running)"