chore: remove unused System.ServiceModel.Syndication dependency and add infra/ to Solution Items

[artcava]

Context

During a code audit, System.ServiceModel.Syndication was found to be referenced in src/XPoster.csproj and documented in the README under Technologies → Utilities, but no usage exists anywhere in the codebase — no using directive, no class instantiation (SyndicationFeed, SyndicationItem, etc.).

Additionally, the infra/ folder (containing README.md, main.bicep, main.bicepparam) is not yet included as a Solution Folder in XPoster.sln, unlike docs/ and the root markdown files which are already present.

Tasks

• Remove <PackageReference Include="System.ServiceModel.Syndication" ...> from src/XPoster.csproj
• Remove System.ServiceModel.Syndication entry from the Technologies → Utilities section in README.md
• Add a new infra Solution Folder in XPoster.sln containing:
  • infra\README.md
  • infra\main.bicep
  • infra\main.bicepparam

Acceptance Criteria

• dotnet build passes after the PackageReference removal
• README.md Technologies section no longer references System.ServiceModel.Syndication
• The infra folder is visible in Visual Studio Solution Explorer under a dedicated Solution Folder

[github-actions]

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