chore(agent): 新增变更日志/checkpoint skill 与 post-commit hook

- AGENTS.md 新增变更日志 Skill 规范：变更后增量更新 document/work-log，更新前先做 Git 拉取检查
- 新增 .codex/skills 下 agent-change-log、git-checkpoint-commit 两个 skill
- 新增 .githooks/post-commit 与 tools/agent-change-log 自动化脚本，提供提交后最低限度日志追加

.githooks/post-commit

@@ -0,0 +1,9 @@
+#!/bin/sh
+# Auto-append a minimal X-Financial agent work-log entry after each commit.
+
+repo_root="$(git rev-parse --show-toplevel 2>/dev/null)" || exit 0
+cd "$repo_root" || exit 0
+
+python3 tools/agent-change-log/update_change_log.py \
+  --event "post-commit hook" \
+  >/tmp/x-financial-agent-change-log-hook.log 2>&1 || true