feat(notifications): top-N contributor summary section for the maintainer recap (public-safe)

[JSONbored]

Digest content section (#1963 'top-N contributor summary, no scoring/reward internals'): a leaderboard of the window's most-merged contributor logins with merged-PR counts only — NO scoring, reward, or trust internals. Each login + line MUST pass isPublicSafeText (src/signals/redaction.ts:48) before emitting, matching the public-safe framing the notifications service already enforces (src/notifications/service.ts:38).

Deliverables

• buildTopContributorsRecapSection(report, limit) → sorted [{login, merged}] capped at limit, ties broken by login
• Reject any line failing isPublicSafeText (src/signals/redaction.ts:48); test that a reward/score-bearing string is dropped
• Unit test: sorting, tie-break, the cap, and the public-safe gate (both pass and reject branches)

References

• src/signals/redaction.ts:48
• src/notifications/service.ts:38
• src/review/alerts.ts:158

Part of #1963.

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size: S · held for later release (gittensor:feature once activated)

[linear-code]

JSO-2345