feat(models): expose legacy inventory as JSON

[tang-vu]

Summary

• add --list --json to the legacy model-directory upgrade helper
• emit the current model plus sorted inventory entries with name, disk size, and active state
• represent an unset current model as JSON null
• preserve the existing human inventory unchanged
• validate the public CLI against a two-model fixture

Why this matters

Older ODS installations still use this production helper for directory-based model upgrades. Automation and migration tooling can now inspect available and active models without scraping colored table output before deciding whether to invoke an upgrade.

Overlap check

Searched open/closed PR titles for upgrade model json list and bodies referencing ods/scripts/upgrade-model.sh. PRs #2613, #2404, and #2266 change compose/state/env mutation paths; none changes the read-only inventory command. This PR is isolated to cmd_list and its CLI option.

Test plan

• bash ods/tests/test-upgrade-model-list-json.sh
• bash -n ods/scripts/upgrade-model.sh
• bash -n ods/tests/test-upgrade-model-list-json.sh
• git diff --check

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