write-chapter: allow Unicode box-drawing inside fenced diagrams

[yagop]

Follows the commit-hook change that now permits non-ASCII inside ``` fenced blocks. This relaxes the matching skill convention so generated chapters can draw cleaner diagrams.

What changed in SKILL.md

• Appendix B, punctuation rule: "ASCII punctuation only" → "ASCII punctuation in prose and code", with an explicit exception: inside a fenced diagram you may use Unicode box-drawing and arrow characters. Everything outside a fence stays ASCII.
• Appendix B, visual-aids rule: "AT MOST one small ASCII diagram" → "one small diagram", with guidance to prefer Unicode box-drawing inside a fence (ASCII art still fine), line connectors up under the node they descend from, give every descending line an arrowhead, and render the page to check it.
• Appendix A, review checklist: the "Non-ASCII punctuation" item is now scoped to outside fenced diagrams, and a new item says to render the page and eyeball the diagram for alignment / missing arrowheads — the exact defect behind the chapter 3 and chapter 5 flowcharts.

The skill text itself stays pure ASCII (it describes the chars by name, doesn't embed them), so the convention is just guidance — nothing about existing chapters changes.

Verification

• SKILL.md has zero non-ASCII bytes (so this commit passed the hook normally).
• bunx tsc --noEmit clean and bun x vitepress build passes — existing chapters (all ASCII diagrams) are unaffected.

Note: this is the convention/guidance side. The enabling change is in the global commit hook (~/.claude/hooks/check-commit-chars.mjs), which now skips the blocklist for lines inside ``` fences — that's not in this repo, so it's not part of this PR.

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