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

tools/agent-change-log/install_post_commit_hook.sh

@@ -0,0 +1,13 @@
+#!/bin/sh
+# Install the versioned agent change-log post-commit hook into this checkout.
+
+set -eu
+
+repo_root="$(git rev-parse --show-toplevel)"
+hook_path="$(git rev-parse --git-path hooks/post-commit)"
+
+mkdir -p "$(dirname "$hook_path")"
+cp "$repo_root/.githooks/post-commit" "$hook_path"
+chmod +x "$hook_path"
+
+printf 'installed_hook=%s\n' "$hook_path"