Extend sandboxing to package processing

[MikeMcQuaid]

• Limit downloaded package operations to their intended output paths and share sandbox setup with build, test and installation workers.
• Preserve required download authentication, validate trust boundaries and warn when package processing proceeds without a sandbox.
• Explain sandboxing principles and limitations in the security guide.

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• Have you followed our Contributing guidelines?
• Have you checked for other open Pull Requests for the same change?
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• Have you written new tests (excluding integration tests)? Here's an example.
• Have you successfully run brew lgtm (style, typechecking and tests) locally?

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• I did not use AI/LLM to create this PR, or I disclosed the tool/model below and reviewed its output; I did not attribute commits to AI and will answer maintainer questions and review comments myself without AI/LLM.

Codex with GPT 6 Astra at Extra High effort, with local review and testing.

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