feat: get and describe commands (Phase 2 kubectl-style CLI)

[robbycochran]

Summary

Phase 2 of the kubectl-style CLI refactor. Adds get and describe commands.

• harness get agents -- list running sandboxes (aliases: sandboxes, sandbox)
• harness get providers -- list registered providers
• harness get gateways -- list gateways (aliases: gateway, gw)
• harness describe <name> -- detailed status for a specific sandbox
• -o table|json|yaml on all get subcommands (table is default)
• Shared OutputFormat type in cmd/output.go
• Credential values never included in structured output
• status deprecation message updated to point to get agents

$ harness get gateways -o json
[
  {
    "name": "openshell",
    "endpoint": "https://127.0.0.1:17670",
    "active": true
  }
]

Test plan

• go build ./... passes
• go test ./... passes (all packages)
• go vet ./... passes
• Local integration --ci: 14/14 passed
• harness get agents/providers/gateways works with all output formats
• harness describe <name> shows sandbox details
• CI workflows green