#3989 Docs: extension quality scores with the registry formula and reports grade (swamp-club#3845)

swamp-club#3845 changes swamp extension quality so the CLI and the registry share one grading system:

• percentage always divides by the rubric's full maximum (16 in rubric v5), the registry's formula. A provisional factor (repository-verified before publish) earns 0 and its points are reported as provisionalPoints; they no longer shrink the divisor. A fully earned extension with a pending repository check now shows 14/16 = 87% (B), not 14/14 = 100%.
• --json gains grade (A at 90 and above, B 75, C 60, D 40, F below 40), right after percentage, computed from percentage with the package's gradeForPercentage.
• The text rubric line reads: Rubric v5 — 14/16 points (87%, grade B, +2 pending server verification: 100% (A) if the registry verifies them, all factors earned).
• maxClientEarnablePoints stays in the JSON but no longer feeds the percentage.

Manual pages that describe the old behavior:

• content/manual/reference/extensions/scorecard-rubric.md — Provisional factors in the CLI (the divisor excluding provisional points).
• content/manual/reference/extensions/publishing.md — the quality JSON field table (add grade; maxClientEarnablePoints meaning) and the sample output line (14/14 points (100% ...)).
• content/manual/how-to/extensions/improve-score.md — repository-verified provisional section (excluded from the score).
• content/manual/tutorials/publish-an-extension.md — the sample rubric line and the pending-verification explanation.

Update once the swamp change ships.