fix: prevent shell options from environment variables

[andrewcodess]

Fixes #1435.
This change prevents ZX_SHELL, ZX_PREFIX, and ZX_POSTFIX from being used to configure shell execution through environment variables.
Changes
Removed shell, prefix, and postfix from ENV_OPTS in src/core.ts.
Updated docs/cli.md to document the security restriction.
Removed the ZX_SHELL environment-variable example from docs/shell.md.
Existing CLI and JavaScript API configuration remains available.
Testing
The relevant behavior was manually verified:
ZX_PREFIX is no longer loaded.
ZX_POSTFIX is no longer loaded.
ZX_SHELL is no longer loaded from the environment.
Other supported environment variables continue to work.
The full local test/pre-push suite could not be completed in my Windows environment due to environment-specific failures in the size, license, and circular-dependency checks.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.