plugin-runner: Handle missing .env file and allow writable target

[ZoogieZork]

Description

• Properly handle the case where no .env file exists. Before, this would cause the script to exit immediately.
• Add a new run-writable subcommand to support plugins that expect the target directory to be writable.

Motivation and Context

Fixes & features from feedback.

How Has This Been Tested?

Tested locally.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation change

Checklist

• My code follows conforms to the coding standards.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.