feat(cli): add --json output for list and status commands

[kjw3]

Problem

Operational commands only emit human-formatted output. That makes automation, scripting, and debugging harder than necessary.

Scope

Add --json output to:

• nemoclaw list
• nemoclaw status
• nemoclaw <name> status
• nemoclaw <name> policy-list

Acceptance Criteria

• Each command above accepts --json
• JSON shape is documented and stable enough for scripting
• Human-readable output remains the default
• Tests validate JSON output shape and exit behavior

Context

This came out of a CLI review against https://clig.dev/. Structured output is the clearest machine-friendliness gap in the current CLI.