docs: clarify boilerplates-ref checkout order

[kitsuyui]

Summary

• Clarifies that boilerplates-ref checkout happens after gibo update only when update: 'true' is used.
• Documents that update: 'false' checks out from the boilerplates database already present on the runner.
• Keeps the README input table and pinning guidance aligned with action.yml.

Verification

• actionlint .github/workflows/*.yml
• typos action.yml README.md
• YAML parse for action.yml and workflow files
• Extracted composite shell payload from action.yml and ran bash -n -
• git diff --check
• git status --short clean