fix: clarify Mermaid line breaks in flowchart labels: \n => <br/>

[iqdoctor]

Summary

• add explicit rule in SKILL.md to use <br/> for multi-line flowchart node labels
• document why escaped \n should be avoided (it renders as literal text in HTML output)
• add wrong/right example in references/libraries.md

Why

Generated diagrams occasionally contained literal \n in node text. This happened because the skill docs discussed <br/> support, but did not explicitly forbid \n in flowchart labels.

Scope

Docs-only change. No runtime code changes.

[iqdoctor]

befoure

after