Add --json to the list commands (alias ls, class ls, watch ls)

[gsjonio]

What & why

alias ls, class ls, and watch ls print human tables only. A --json variant makes them scriptable, matching scan --json and keeping the CLI consistent.

Where

• cmd/netwp/cmds.go (the runAlias/runClass/runWatch list branches) and cmd/netwp/json.go.

Scope

• When hasArg("--json"), emit the list as a JSON array (MAC, name/class, etc.) via printJSON.
• Small command-local DTOs if the store types don't serialize cleanly (see how scan --json uses a DTO).
• Document in help text and both READMEs.

Acceptance

• netwp alias ls --json (and class/watch) emit valid JSON.
• Full verify suite clean.

Good first issue: repetitive, guided by the existing --json commands.