Extend --output json to sandbox list, sandbox create, and status commands #1964

Problem Statement

sandbox get and status output human-readable colored text with no machine-readable format option. Programmatic callers (Go, Rust, shell scripts) must parse ANSI-colored output and strip escape codes to extract sandbox details and gateway status.

sandbox list and provider list already support --output json/yaml/table via the OutputFormat enum. sandbox get and status do not.

Remaining Gaps

Proposed Design

Add --output json/yaml to sandbox get and status using the existing OutputFormat enum and sandbox_to_json helper.

sandbox get --output json:

{"id":"abc-123","name":"happy-fox","phase":"Ready","labels":{},"resource_version":42,"created_at":"2026-06-21 10:00:00","current_policy_version":2}

status --output json:

{"gateway":"podman-test","server":"http://127.0.0.1:8090","status":"connected","version":"0.0.66"}

Alternatives Considered

1. Use the Python SDK — Works for Python callers. The SDK returns structured SandboxRef objects for get and list.
2. Parse CLI output — Current approach for non-Python callers. Works but fragile.

Agent Investigation

• sandbox get renders at run.rs:2535 with println! calls, no format flag
• status renders at run.rs:507 with println! calls, no format flag
• OutputFormat enum and print_output_collection helper in output.rs are reusable
• sandbox_to_json() at run.rs:3352 already serializes sandbox objects to JSON