feat: add article Markdown output

[ryanfowler]

Summary

• add --article to extract readable HTML with Legible and convert it to Markdown with htmd
• emit YAML frontmatter from extracted article metadata and the final response URL
• pass existing Markdown responses through with URL-only frontmatter
• preserve output formatting, pager, clipboard, and file behavior while adding clear unsupported-content diagnostics
• document the HTML-to-Markdown workflow and add unit/integration coverage

Validation

• cargo fmt --check
• cargo clippy --locked --all-targets --all-features -- -D warnings
• cargo test --locked --all-features --lib --bins -- --test-threads=1
• cargo test --locked --all-features --test cli --test formatting

Caveat

• Prettier was unavailable in the environment; documentation changes pass git diff --check.