chore: Remove dead/unused NuGet packages

## What

Remove NuGet packages declared in `Directory.Packages.props` that have zero project references and zero code usage. Verified by grepping `PackageReference Include=""` across all `.csproj`/`.fsproj` files.

## Packages to remove

| Package | Version |
|---------|---------|
| `MessagePack` | 2.5.302 |
| `KubernetesClient` | 19.0.2 |
| `Riok.Mapperly` | 4.2.1 — Generator.Equals already handles equality codegen |
| `Samboy063.Tomlet` | 6.0.0 — second TOML parser; Tomlyn is the one actually used |
| `TUnit` | 0.25.21 — suite runs entirely on xunit.v3 |
| `Github.Actions.Core` | 9.0.0 |
| `Amazon.Lambda.APIGatewayEvents` | 2.7.1 |
| `Amazon.Lambda.AspNetCoreServer.Hosting` | 1.9.0 |
| `Aspire.Hosting` | 13.4.3 — bare meta-package; only `Aspire.Hosting.AppHost` is referenced |

## How

Delete each `` entry from `Directory.Packages.props`. Run `dotnet build` to confirm nothing breaks.

Part of the broader ponytail over-engineering audit tracked in #3574.