Find body sections in a GOV.UK content item matching a pattern.
Returns a list of `{anchor, heading, snippet, match}` hits — small per-section
snippets centred on the match — so the LLM can decide which full sections to
read via govuk_get_section.
Use this when answering content-based questions ("what does this guide say
about X?", "find the bit about eligibility") rather than navigating by
section number.
Pattern is regex; if it doesn't compile, falls back to literal substring.
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