[codex] Preserve imported front matter titles

[KSemenenko]

Summary

• preserve explicit front matter titles for text-based shell imports
• keep file-stem title fallback for converted non-text imports
• add a core regression test for markdown imports with front matter

Root Cause

The shell import flow built the correct descriptor first, then unconditionally overwrote the imported title with the picker/file-stem fallback.

Validation

• dotnet test @./tests/dotnet-test-progress.rsp --project ./tests/PrompterOne.Core.Tests/PrompterOne.Core.Tests.csproj
• dotnet test @./tests/dotnet-test-progress.rsp --project ./tests/PrompterOne.Web.UITests.Shell/PrompterOne.Web.UITests.Shell.csproj
• dotnet format ./PrompterOne.slnx
• dotnet build ./PrompterOne.slnx -warnaserror