`miren sandbox list --json` bypasses sensitive env-var handling

Running miren sandbox list --json unexpectedly returns each sandbox’s full specification, including resolved service environment-variable values.

That is surprising for a list command. The normal output reads like sandbox inventory, and --json suggests a machine-readable version of that inventory. It should not silently become a raw spec dump with substantially more sensitive data.

This also sidesteps the env-var handling consolidated in MIR-1356, where miren env became the intentional, masked surface for inspecting values. In the observed output, cluster-backed secret references remained opaque, but manually supplied values appeared resolved.

miren sandbox list --json should expose no more sensitive information than the human-readable list. It could omit environment values entirely or return keys and redacted references. If raw sandbox specs are needed for debugging, that should be a separately named, deliberately authorized operation whose sensitivity is clear.

The text and JSON forms should ideally serialize the same purpose-built list representation rather than having JSON serialize the underlying sandbox entity.