Unify scoped language capability counts

[Widthdom]

Summary

• build CLI and MCP language rows and summaries from one shared runtime capability catalog
• expose machine-readable catalog, matched_catalog, and indexed_workspace counts with explicit scope/capability labels
• preserve filter, pagination, alias, and workspace plugin behavior while separating unavailable, empty, and partial index states
• document the contract in the English and Japanese user/testing sections and add the bilingual changelog fragment

Validation

• dotnet restore CodeIndex.sln
• dotnet build CodeIndex.sln -c Release --no-restore -p:UseSharedCompilation=false (0 warnings, 0 errors)
• full net8.0 suite: 10,950 passed, 7 skipped, 0 failed
• full net9.0 suite: 10,476 passed, 420 target-specific skipped, 0 failed
• latest-main language regression: 649 passed on net8.0 and 649 passed on net9.0
• dotnet run --project tools/CodeIndex.Changelog -- check
• Codex adversarial review: No blocking/actionable issues found.

Follow-up candidates

None.

Fixes #4895