Add MIT license metadata to all NuGet packages

[autocarl]

Context

An external consumer review found that the Repl NuGet packages do not currently declare license metadata even though the repository is licensed under MIT:

The NuGet packages declare no license metadata (the repo is MIT) — can you add PackageLicenseExpression to the packages so consumers aren't looking at "no license" on nuget.org?

Source: erikdarlingdata/PerformanceStudio#388 (review)

The review specifically encountered Repl, Repl.Mcp, and Repl.Testing, but the repository contains additional packable projects that should expose the same metadata consistently.

Proposed change

Declare the MIT SPDX expression centrally in src/Directory.Build.props, for example:

<PackageLicenseExpression>MIT</PackageLicenseExpression>

This should apply to every packable Repl project without duplicating metadata across individual .csproj files.

Acceptance criteria

• Every generated Repl .nupkg declares the MIT license using NuGet license-expression metadata.
• At minimum, Repl, Repl.Mcp, and Repl.Testing no longer appear as having “no license” to NuGet consumers.
• The packed .nuspec files contain <license type="expression">MIT</license>.
• The existing Release pack/build validation remains green.