feat: Add auto-detection of ArticleID in yt articles fetch (#641)

[ryancheley]

Summary

Make the article_id argument optional in the yt articles fetch command and enable auto-detection from markdown files containing an <!-- ArticleID: XXX --> comment.

This improves UX by allowing users to fetch articles by simply specifying the local markdown file, without needing to remember or provide the article ID.

Changes Made

• Make article_id argument optional in fetch command
• Auto-detect article ID from markdown file's ArticleID comment
• Add proper error handling for missing article ID
• Add 3 comprehensive tests for auto-detection feature
• Update documentation with new behavior and examples
• Update CHANGELOG with feature description

Examples

# Auto-detect article ID from file
yt articles fetch --file my-article.md

# Still works with explicit ID
yt articles fetch DOCS-A-1 --file my-article.md

# Search current directory for file with ArticleID
yt articles fetch

Test Plan

• ✅ Unit tests added and passing (3 new tests)
• ✅ All existing tests still pass (1362 tests)
• ✅ Pre-commit hooks pass
• ✅ Type checking passes
• ✅ Documentation build succeeds

Documentation

• Updated docs/commands/articles.rst with fetch command documentation
• Updated CHANGELOG.md with feature description

Fixes #641