docs: add Autohand Code install notes

[igorcosta]

Hi, thanks for publishing Serenity.skill. I noticed the README already documents several Agent Skills-compatible runtimes, so this adds Autohand Code as another install target without changing the skill package itself.

What changed:

• add Autohand Code user-level install commands using ~/.autohand/skills/serenity-skill
• add project-level install commands using .autohand/skills/serenity-skill
• mention the autohand --skill-install serenity-skill form only for a future/cataloged Autohand Skills index entry, so readers are not told that catalog install is available before it is published there
• keep the English and Chinese README variants aligned

Validation:

• git diff --check
• python scripts/validate_skill.py .

One local note: I ran the validator from a checkout directory named serenity-skill, because the script checks that the parent folder name matches the SKILL.md frontmatter name.