Support --json in `openshell policy get`

[jacobtomlinson]

Problem Statement

I want to be able to check for specific values in my policy easily. If I run openshell policy get <sandbox> --full I can get a YAML dump of the policy, but it would be more convenient to dump it in JSON so that I can use jq to filter and transform the output.

Proposed Design

Add an additional flag to select json output. Either --json to be consistent with other commands, or --output/-o json to be consistent with other tools like kubectl.

Alternatives Considered

I could use yq to filter the YAML, but it's not installed by default on many systems and would increase the complexity of instructions if users need to install it. jq is a much more standard tool.

Agent Investigation

No response

Checklist

• I've reviewed existing issues and the architecture docs
• This is a design proposal, not a "please build this" request