Add endpoint override for aevatar

[eanzhao]

Summary

• add a per-invocation --endpoint override to aexon aevatar
• thread the override through REPL, one-shot chat, config display, and chat / web startup
• document the new usage and cover the new parsing paths with unit tests

Testing

• dotnet test tests/Aexon.Core.Tests/Aexon.Core.Tests.csproj --filter "FullyQualifiedName~AevatarCommandHelpersTests|FullyQualifiedName~AevatarWebFlagsTests|FullyQualifiedName~AevatarChatSettingsStoreTests"
• dotnet run --project src/Aexon.Cli -- aevatar --endpoint not-a-url