fix(llms): preserve heading section in llms-hidden conditional content

[cderv]

When a heading is the first element inside ::: {.content-hidden when-format="llms-txt"} in a website with llms-txt: true, the heading renders in HTML as a bare <h2> with no <section id="..."> wrapper. The auto-generated TOC link points at an id that no longer exists, so clicking it does not scroll, and the heading's anchor and cross-references break too.

Root Cause

The llms conditional-content filter marks "visible in HTML, hidden from llms" blocks by wrapping them in <div class="llms-hidden-content">. When that block leads with a heading, Pandoc's --section-divs does not keep the heading nested in the div — it fuses the marker class onto the <section> it generates for the heading:

<section id="conditional-section" class="level2 llms-hidden-content">

cleanupConditionalContent in src/project/types/website/website-llms.ts then unwraps every .llms-hidden-content element. Since the class now rides on the <section>, unwrapping strips the section and its id, leaving the bare <h2>. Only the leading-heading shape is affected; with text before the heading or multiple headings, Pandoc keeps the wrapper div separate, so the unwrap is harmless.

Fix

When the marked element is a <section> (the marker fused onto it), keep the section and drop only the llms-hidden-content class; genuine wrapper divs are still unwrapped. Content stays excluded from the .llms.md output, which is generated before cleanup runs.

Test Plan

• Render an llms-txt website with a heading inside content-hidden when-format="llms-txt" — heading keeps its <section id> and the TOC link scrolls
• .llms.md still excludes the HTML-only content
• content-visible when-format="llms-txt" (llms-only) content stays absent from HTML, present in .llms.md
• section-divs: false documents still render the heading with its id

Fixes #14562

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