Add LabelIndex page

[prozolic]

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[Copilot]

Pull request overview

Adds a cross-archive label index and per-label PR list pages to the generated static site. The generator now scans every archived markdown file, aggregates PRs by label using the existing PullRequestAnalyzer, and emits outputs/labels/index.html plus outputs/labels/{sanitized-label}/index.html pages. The index page also gets a new link into the label index, and a reusable scroll-to-top button is introduced for long pages.

Changes:

• New CreateLabelPageHtml pipeline step and LabelPullRequestInfo aggregation type in Program.cs, plus the centralized StringComparerOptions.DefaultComparer.
• New GenerateLabelIndexHtml, GenerateLabelPageHtml, SanitizeLabelForPath, and supporting CSS/JS (sortable table, scroll-to-top button) in HtmlGenerator.cs; GenerateIndex now filters output to four-digit year directories and links to the label index.
• Shared StringComparer extracted into StringComparerOptions and reused from Program and HtmlGenerator.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/PRDigest.NET/StringComparerOptions.cs	Introduces a shared NumericOrdering invariant StringComparer reused by multiple call sites.
src/PRDigest.NET/Program.cs	Adds CreateLabelPageHtml (aggregating labels across all archives) and the LabelPullRequestInfo type; replaces inline comparer with the shared one.
src/PRDigest.NET/HtmlGenerator.cs	Adds label index/per-label page generators, label-path sanitizer, sortable-table script, scroll-to-top button/CSS, and tightens GenerateIndex to only enumerate four-digit year dirs.

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