Add JSON output for basectl check

[codeforester]

Summary

• Add basectl check --format json
• Preserve the default human-readable logging behavior on stderr
• Emit structured check results to stdout for automation
• Keep existing exit-code semantics:
  • 0 when all checks pass
  • 1 when any requirement is missing
• Validate unsupported output formats
• Mark the TODO item complete

JSON Output

Example:

{
  "ok": true,
  "checks": [
    {"name":"homebrew","ok":true,"message":"Homebrew is installed."},
    {"name":"xcode_command_line_tools","ok":true,"message":"Xcode Command Line Tools are installed."},
    {"name":"python","ok":true,"message":"Python formula 'python@3.13' is installed via Homebrew."},
    {"name":"bats","ok":true,"message":"BATS formula 'bats-core' is installed via Homebrew."},
    {"name":"base_virtualenv","ok":true,"message":"Virtual environment exists at '/Users/rameshhp/.base.d/base/.venv'."}
  ]
}