fix(viewer): exclude log.md from concept nodes

[PranaliPathak04]

#202 Issue

Summary

• Exclude reserved log.md files from viewer concept nodes.
• Add a regression test ensuring both index.md and log.md are excluded.

Motivation

The OKF specification defines both index.md and log.md as reserved filenames, but the viewer only excluded index.md when walking Markdown files.

This change brings the viewer behavior in line with the specification.

Testing

• pytest tests/test_viewer.py
• 7 tests passed
• git diff --check