feat(agent-skills): add Claude Code skill bootstrap

[shunk031]

Summary

• Add a shared agent skill bootstrap script for globally installing Skill Creator for Claude Code.
• Wire the script into chezmoi as a run-once common bootstrap step.
• Add focused Bats coverage for the installer command shape and future Codex extension point.

Validation

• bash -n install/common/agent_skills.sh
• shfmt --indent 4 --space-redirects --diff install/common/agent_skills.sh tests/install/common/agent_skills.bats
• git diff --check
• shellcheck install/common/agent_skills.sh
• Stubbed mise / skills execution smoke test

Note: Bats was not run locally per repository policy.