CommandRegistrar.parse_frontmatter located the closing delimiter with
content.find("---", 3), a raw substring search. It stopped at the first
"---" anywhere after the opening — including one embedded in a
frontmatter value (e.g. a description "Separate sections with ---
markers") or inside an indented literal block — which truncated the
frontmatter and spilled the remainder into the body, silently corrupting
both the parsed metadata and the rendered command body.
Match the closing "---" on line boundaries, mirroring the line-anchored
scan already used by VibeIntegration._inject_frontmatter_flag.
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