This commit renames the `isolated` devcontainer to `hardened` to more accurately reflect its purpose of providing a security-hardened environment rather than complete network or filesystem isolation.

The following changes have been made:

- Renamed the `.devcontainer/isolated` directory to `.devcontainer/hardened`.
- Updated all references to the `isolated` container in `devcontainer.json` files, `README.md`, and `.github/workflows/main.yml` to use the new `hardened` name.
- Standardized the terminology in comments and descriptions to use "hardening" instead of "isolation" where appropriate.
- Added security warnings to the `README.md` to clarify the security implications of using devcontainers.

This change improves the clarity of the projects documentation and configuration.