Fix index console output and completion UX

[Widthdom]

Summary

• Tighten console width fallback handling and add verbose diagnostics for width detection failures.
• Use invariant numeric formatting for index progress/summaries and document stable CLI output formatting.
• Add configurable completion notifications for long human index/update runs.
• Show first-run next-step guidance after a successful full index.

Issues

Fixes #1629
Fixes #1662
Fixes #1835
Fixes #1963

No target issue had an existing work-started comment, so none were excluded.

Validation

• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release --no-build --filter "FullyQualifiedNameConsoleUiTests|FullyQualifiedNameIndexCommandRunnerTests.ParseArgs_NotifyFlag" -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release --no-build --filter "FullyQualifiedName~FileIndexerTests.DetectLanguage_WorkspaceLangMapYaml_AliasesExtension" -p:UseSharedCompilation=false
• codex exec adversarial review: No blocking/actionable issues found.

Full Release solution tests were attempted with --no-build; they ran for an extended period with no reported failures but did not produce a usable completion summary in this environment.