fix(packaging): declare MIT license expression

[autocarl]

Summary

• declare the repository's MIT license as a NuGet PackageLicenseExpression
• centralize the metadata in src/Directory.Build.props so every packable Repl project inherits it
• ensure NuGet consumers no longer see the published packages as having no license

Closes #68

Validation

• dotnet restore src/Repl.slnx
• dotnet pack src/Repl.slnx -c Release --no-restore -p:WarnOnPackingNonPackableProject=false
  • inspected all 10 generated .nupkg archives
  • verified every .nuspec contains <license type="expression">MIT</license>
• dotnet build src/Repl.slnx -c Release -warnaserror --no-restore
  • 0 warnings, 0 errors
• dotnet test --solution src/Repl.slnx -c Release --no-build --no-restore
  • 1,312 passed, 0 failed, 1 skipped (opt-in MCP Inspector external-toolchain smoke)

[autocarl]

CI note: Build, Test, Pack is red because the dorny/test-reporter step cannot create a check for this fork PR (Resource not accessible by integration). The job's Restore, Build, and Test steps all succeeded; Pack was then skipped because the reporter step had failed.

I validated the exact GitHub merge tree (db43461275572ad8cc2362532bbe67aafe93690d) locally:

• dotnet pack src/Repl.slnx -c Release --no-restore -p:WarnOnPackingNonPackableProject=false
• 10 .nupkg archives produced and inspected
• all 10 .nuspec files contain <license type="expression">MIT</license>
• strict Release build: 0 warnings, 0 errors
• full test suite: 1,312 passed, 0 failed, 1 opt-in external-toolchain smoke skipped

Failing job: https://github.com/yllibed/repl/actions/runs/29460470963/job/87502690109