Fix {Description} placeholder not replaced in non-content-item page templates

[Copilot]

Non-content-item templates (Search Pages, Contact, Archive, etc.) that use {Description} would emit the literal placeholder rather than a meaningful value — e.g., <meta name="description" content="{Description}">.

Changes

• TemplateProcessor.cs — Added {Description} → pageTitle replacement in ProcessNonContentItemTemplate. For Search Pages, pageTitle is "Tag: {tag}", consistent with how {Title} is already populated from the same value.
• TemplateProcessor_ProcessNonContentItemTemplate_Should.cs — Added ReplaceTheDescriptionPlaceHolderWithThePageTitle test asserting both presence of the resolved value and absence of the raw {Description} token.

// Before
return template.Content
    .Replace("{Title}", pageTitle)
    .Replace("{ByLine}", String.Empty)
    .ReplaceNonContentItemSpecificVariables(...);

// After
return template.Content
    .Replace("{Title}", pageTitle)
    .Replace("{Description}", pageTitle)   // ← added
    .Replace("{ByLine}", String.Empty)
    .ReplaceNonContentItemSpecificVariables(...);