Keep default values out of global option help

[autocarl]

Summary

Addresses review feedback on #34 by keeping the PR scoped to global option descriptions in help output, without introducing default-value rendering.

• Removes [default: ...] rendering from root global option help.
• Keeps implicit CLR defaults (false, 0, etc.) from being stored as registration defaults, so caller fallbacks still work for value-typed globals.
• Keeps the public API compatibility fix from Fix global option descriptions in help output #34: existing AddGlobalOption(...) overloads remain, and description-capable overloads are additive.
• Updates integration tests to assert descriptions render and default values are not introduced by this PR.
• Adds accessor regression coverage for value-typed globals registered without explicit defaults, including the description overload.

Follow-up

Created #40 for a dedicated design/implementation of global option default-value rendering, including [DefaultValue] support.

Validation

• dotnet test src/Repl.Tests/Repl.Tests.csproj -c Release --filter 'FullyQualifiedName~Given_GlobalOptionsAccessor' — 40 passed
• dotnet test src/Repl.IntegrationTests/Repl.IntegrationTests.csproj -c Release --filter 'FullyQualifiedName~Given_CustomGlobalOptions' — 5 passed
• dotnet build src/Repl.slnx -c Release -warnaserror --no-restore — 0 warnings/errors
• dotnet test src/Repl.slnx -c Release — 982 passed
• npx -y markdownlint-cli2 '**/*.md' — 0 errors
• git diff --check

[chatgpt-codex-connector]

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[autocarl]

Updated after Codex review: 5bbfac5 preserves caller fallbacks for value-typed global options registered without explicit defaults, adds regression coverage for both generic overloads, and still keeps default-value rendering out of help. Local validation is updated in the PR body.