fix: preserve <br> line breaks in fenced code

[matlec]

@phil-abb's review comment on my PR revealed an issue with how we handle <br> when assembling content for the site.

Summary

The content sync deletes every <br> from Markdown before the MDX build (sed 's/<br>//g'). Inside mermaid diagrams a <br> is a real line break, so a label like GET /api/v1/deployments followed by Header: If-None-Match: "sha256:abc..." renders as one run of text on the site.

Change

Replace the blanket delete with a pass that depends on context. Inside fenced code, self-close <br> so mermaid keeps its breaks. Everywhere else, strip it as before: a bare <br> is invalid JSX, and in a heading it becomes a raw node that crashes fumadocs' rehypeToc. This also drops the macOS/Linux sed -i split, since awk is portable.

Verification

I ran the sync and dev server against a branch that uses <br> in a mermaid diagram. The diagram renders with line breaks.

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