Add Observables.Samples.Mqtt

[Skymly]

Summary

• Add console sample for Observables.Mqtt R3 topic proxy factory registration (no broker in CI).
• Wire slnx, Nuke Ci (skip Mqtt when UseLocalObservables=false), Directory.Build targets, and README.

Test plan

• dotnet build Observables.Samples.slnx -c Release
• dotnet run --project build/_build.csproj -- --target Ci
• dotnet run --project Observables.Samples.Mqtt -p:UseLocalObservables=true