Defensive: when the generator encounters a fenced code block containing
Unicode box-drawing characters, wrap it in `<!-- ascii-guard-ignore -->`
markers so the docs-site-checks lint (which scans inside code fences)
can't reject the page for a skill's own diagram.
Plain bash/python code blocks stay uncluttered — only blocks with box
chars get wrapped. Skill authors no longer have to remember to add the
ignore markers in every SKILL.md with ASCII art.
Fixes#15305.
