Add JSON output for agent status

[phucnguyen1707]

Summary

• add moshcode agents --json for machine-readable engine status
• support the same JSON view in the TUI
• expose the option consistently in Bash, Zsh, Fish, and PowerShell completions
• preserve agents <engine> --json passthrough behavior

Testing

• npm test (1264 tests, 1049 passed, 215 skipped, 0 failed)
• focused CLI, completion, and TUI tests
• git diff --check