Docs: sharpen SecGate positioning and accuracy

[danieloceno]

Summary

• align the README with the Stelnyx product-family voice
• clarify installed/enabled scanner behavior and exit codes
• document network use by Semgrep and vulnerability scanners accurately
• qualify deterministic aggregation versus changing external advisory data
• separate shipped local CLI capabilities from planned commercial tiers
• update the report schema example to v0.2.14

Validation

• npm run lint
• git diff --check

Documentation-only change.