Add 'pm2 summary' command for concise process overview

Feature Request: pm2 summary Command

Overview

Add a new CLI command, pm2 summary, to provide a concise single-line summary of all managed processes. This command should help users quickly understand the state of all their applications without parsing the full, detailed output of pm2 status or pm2 list.

Example Output

3 apps online, 1 app stopped, 0 app errored

Proposed Features

• New CLI command: pm2 summary
• Outputs number of processes by state (online, stopped, errored, etc.)
• Optional --json flag to output summary in JSON format for scripting/automation

Benefits

• Provides a quick, human-readable overview
• Useful for dashboards, scripts, and monitoring tools

Additional Notes

• Should reuse existing process state detection logic where possible
• Should require minimal changes to core architecture

Start by locating the existing pm2 status or pm2 list CLI handlers and the process state detection logic they use. Define how pm2 summary should count each state and how its optional --json output should be structured. Done means the new command provides the concise human-readable overview and JSON summary described in the issue.