fix(sys): gate diagnostic sections behind -v; route json-mode logs to stderr

[timenick]

Summary

• Move Backend SDKs and Export Readiness panels into --verbose; verbose now lists all backends including not-installed rows so the section is informative even when no SDK is installed.
• Route the per-command RichHandler to stderr under --format json so DEBUG/WARNING logs no longer corrupt the JSON payload on stdout.
• Update _run_sys_json to read result.stdout (Click 8.4's result.output still aggregates stderr).

Fixes #736

Before / After

1. Default winml sys — drops two diagnostic panels

Before (on a host with at least one backend SDK installed, the panels show up by default; below is the shape they took):

...
Backend SDKs
  QNN SDK     Installed    <path>
  OpenVINO    Installed    <version>

Export Readiness
  ONNX Export            Ready
  QNN Compilation        Ready
  OpenVINO Conversion    Ready
...

After — those panels are gone from default output; users only see Environment, ML Libraries, Devices, and Execution Providers.

2. winml sys -v — sections are now always populated

After (verbose now lists every backend regardless of install status, so the panel is meaningful on a stock dev box):

PyTorch Details
  CUDA Available    False

Backend SDKs
  QNN SDK     Not found    -
  OpenVINO    Not found    -

Export Readiness
  ONNX Export            Ready
  QNN Compilation        SDK required
  OpenVINO Conversion    Not installed

3. winml sys --verbose --format json — stdout is now valid JSON

Before — RichHandler writes DEBUG records to the same console as the JSON, so stdout starts with log lines and json.loads fails:

[05/26/26 10:35:28] DEBUG    OpenVINO not available
[05/26/26 10:35:29] DEBUG    Found EP: OpenVINOExecutionProvider at ...
                    DEBUG    Found EP: NvTensorRTRTXExecutionProvider at ...
{... JSON would appear here, never reached by parsers ...}

After — stdout contains only JSON; the same log records are emitted on stderr where they belong:

$ winml sys --verbose --format json 2>/dev/null | head
{
  "python": {
    "version": "3.11.14",
    "executable": "C:\\Projects\\WinML-ModelKit\\.venv\\Scripts\\python.exe",
    "implementation": "CPython"
  },
  "platform": {
    "system": "Windows",
    ...