Batch scanner mixes progress messages into JSON stdout #753

The batch scanner includes its heading and progress messages on stdout when --format json is requested. Redirecting that output to a file produces a document that cannot be parsed as JSON.

This affects contrib.batch_scan.batch_scan; the main skillspector scan CLI is a separate entry point.

Reproduce

On main at b6f03047a9acb9b1975aa3787eb72894ef28d2aa, create skills/example/SKILL.md:

---
name: example
description: Organize local notes.
---
# Example
Organize local notes into a checklist.

Run:

python -m contrib.batch_scan.batch_scan skills --no-llm --format json > report.json
python -m json.tool report.json

Expected: report.json contains only the JSON report; progress and warnings go to stderr.

Actual: stdout starts with SkillSpector Batch Scan and a progress line before the JSON object. The JSON parser reports Expecting value. The scan itself can still complete.

Using --output report.json already writes a clean report file. The defect is in stdout routing. Markdown stdout receives the same progress messages, and the Rich printing helper also drops explicit file=sys.stderr arguments.

I have a focused fix and regression tests for report-only stdout, diagnostic routing, Rich/plain printing, and the existing exit codes.