Add HeadingSummary plugin for extracting article headings as navigation metadata

[Copilot]

Implements a new HeadingSummary plugin that parses post HTML and stores a structured list of headings as document metadata, enabling in-page navigation (table of contents, sidebars, etc.).

New plugin: HeadingSummary

• Heading.cs — Data model with Level (int), Id (string?), Text (string?)
• HeadingSummaryModule.cs — ParallelModule using HtmlAgilityPack to select h1–h6 nodes and emit a List<Heading> under the "Headings" metadata key
• HeadingSummaryConfigurator.cs — IConfigurator<Bootstrapper> that inserts the module into the Content pipeline at position 3, matching the ReadingTime pattern

Sample updates

• 1.md — headings promoted to proper markdown with {#anchor-id} syntax
• _sidebar.cshtml — renders Headings as a navigation <ul> with anchor links

Usage

Add headings with IDs in your markdown:

## My Section {#my-section}

Render in a template:

@{
    var headings = Model.GetList<Statiq.Plugins.Heading>("Headings");
}
@foreach (var heading in headings)
{
    <li class="heading-level-@heading.Level">
        <a href="#@heading.Id">@heading.Text</a>
    </li>
}

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• blog.jermdavis.dev
  • Triggering command: /home/REDACTED/work/_temp/ghcca-node/node/bin/node /home/REDACTED/work/_temp/ghcca-node/node/bin/node --enable-source-maps /home/REDACTED/work/_temp/copilot-developer-action-main/dist/index.js (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Heading Summary</issue_title>
<issue_description>Extracting blog article headings for navigation from @ Jeremy Davis

• https://blog.jermdavis.dev/posts/2024/extracting-article-headings</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Heading Summary #3

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