Fix LaTeX dollars

Investigate and fix the actual $ usages in Markdown content. The $ fall into three classes: - Currency (escape these) — $1, $2 billion, R$ 549 → these pairs cause all the warnings - Real math (leave alone) — $\rightarrow$, $O(1)\text{ streaming}$ → valid, no warnings - Shell code (leave alone) — $(curl…), ${ZSH_CUSTOM}, $HOME → inside code blocks Execute in 4 steps: - Investigate — greps the content, classifies every $ into currency / real math / shell code, and reports counts before changi...