Action-specific variable names

[CasperWA]

FIxes #50

Prefixes all local bash variable names with PUSH_PROTECTED_ to avoid clashing with workflow-created variables.

Minor updates:

• Explicitly use string values as default in action.yml.
• Update pre-commit hook black (no updates after running pre-commit run --all-files).
• Silence linting for using HTML in README.md.