Agent sandbox blocks required instruction files and screenshots attached to prompts

## Description
The agent's sandbox blocks access to files that are part of the prompt context, including required instruction files and screenshots attached to a prompt.

One reported example is the required TypeScript instruction file at `/Users/hawkticehurst/.copilot/instructions/migration-demo.instructions.md`, which the agent reported was blocked by the sandbox.

In another case, the agent could not access the file paths pointing to screenshots attached to the prompt.

## Reproduction scenarios
- Run an agent task that requires the instruction file above and have the agent attempt to read it.
- Attach screenshots to an agent prompt and have the agent attempt to access the supplied screenshot paths.

These scenarios describe the reported failures; exact reproduction steps and sandbox configuration have not yet been captured.

## Expected behavior
Required instruction files and explicitly attached screenshots should be accessible to the agent through the supported context/attachment mechanism without disabling sandbox protections.

## Actual behavior
Sandbox restrictions prevent access to these paths, so the agent cannot read required instructions or inspect the attached screenshots.

## Impact
Tasks may proceed without required instructions or cannot use the visual context the user explicitly provided.