#3845 extension quality --json reports percentage but not the grade that extension info and the registry show

On 20261020.024627.0-sha.26dc044f (macOS 27), swamp extension quality <manifest> --json returns percentage, status, earnedPoints, maxEarnablePoints, maxClientEarnablePoints, provisionalPoints, allPassed and factors, but no grade. swamp extension info --json shows score.grade for a published extension scored by the same rubric, and the rubric reference publishes the thresholds (A at 90% and above, B 75, C 60, D 40, F below 40).

Source: the JSON object is built in src/presentation/renderers/extension_quality.ts:398 from the local score with no grade field; grade: string exists only on the registry response type in src/infrastructure/http/extension_api_client.ts:185, which extension info prints.

Expected: the grade the registry will show, in the output that predicts it, so a script can gate on grade the same way it reads percentage. Actual: the caller maps percentage to a letter with the table from the rubric page.

Steps to reproduce:

1. swamp extension quality <manifest> --json | python3 -c 'import json,sys; print(sorted(json.load(sys.stdin)))'
2. grade is absent; percentage and provisionalPoints are present.

Note: provisionalPoints marks points pending server verification (the text output prints +N pending server verification), so a locally computed grade can be a floor on the registry's grade; inference: that may be why the field is left out.

Suggestion: add grade to the quality JSON, derived from percentage with the published thresholds, and say in the reference that it is provisional when provisionalPoints is above zero.