fix(notifications): wire maintainer-recap's top-contributors section builder into the actual digest output

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⚠️ Definition of Done: this issue must be completed in full, in a single PR. Do not split this
work across multiple PRs, and do not defer any Deliverable below to a follow-up issue. A PR that
satisfies only some of the Deliverables, stubs a required test, or leaves a checkbox
partially-done does NOT resolve this issue and will be closed.

Context

Mirror this precedent exactly: #8372, which fixed the identical bug shape for four sibling section
builders (buildCalibrationRecapSection, buildGateOutcomesRecapSection, buildPerRepoRecapSection,
buildDriftRecapSection) — "shipped fully implemented + unit-tested but never composed into the delivered
digest." src/services/maintainer-recap-top-contributors.ts is the fifth sibling in that same family and it
fell through #8372's sweep untouched.

src/services/maintainer-recap-top-contributors.ts exports buildTopContributorsRecapSection (line 42),
described in its own header comment as: "Maintainer-recap TOP-CONTRIBUTORS section (#2244, content slice of
the #1963 recap digest)." It is fully implemented (dedup/rank/cap/public-safe-gate logic complete) and fully
unit-tested (test/unit/maintainer-recap-top-contributors.test.ts).

grep -rn "buildTopContributorsRecapSection" src/ (excluding test/) shows it is imported and called
nowhere — confirmed the same way #8372 confirmed its four siblings were dead code. formatMaintainerRecap
(src/services/maintainer-recap.ts, the function that renders a RecapReport into the Discord/Slack digest
body) now calls all four of #8372's siblings (see the #8372 comment block at src/services/maintainer-recap.ts
lines 17-22 and their unconditional call sites around lines 177-217) but never calls
buildTopContributorsRecapSection, and the emitted digest never contains a "## Top contributors" heading.

Unlike #8372's four siblings, this section needs data RecapReport does not currently carry: a per-login
tally of merged PRs across the scan window. That data source already exists and is already used for an
almost-identical per-login tally elsewhere in this codebase — mirror
buildMaintainerQualityDashboard's contributorTotals pattern
(src/services/maintainer-quality-dashboard.ts lines 93, 142-153): it iterates a repo's pull requests,
tallies a Map<string, {...}> keyed on pr.authorLogin ?? "unknown", then sorts/caps at line 156-160 — the
exact same shape buildTopContributorsRecapSection already expects as its TopContributor[] input.

The raw per-repo merged-PR data to tally already exists via
listRecentMergedPullRequests(env, fullName): Promise<RecentMergedPullRequestRecord[]>
(src/db/repositories.ts line 5090), whose records carry authorLogin and mergedAt
(src/types.ts line ~2081-2091) — no new D1 query or schema change is needed.

runMaintainerRecapJob (src/review/maintainer-recap-wire.ts, around lines 170-215) already loops
repoNames once per digest run and loads two aggregators per repo (loadGatePrecisionReport,
buildRepoOutcomeCalibration) into MaintainerRecapRepoInput[]. This is the natural place to also call
listRecentMergedPullRequests per repo (same loop, same try/catch-per-repo fail-safe pattern already there)
and fold the results into a window-filtered (mergedAt within windowDays), login-tallied contributor list.

Requirements

1. Add a contributors: TopContributor[] (or equivalently-shaped) field to MaintainerRecapInputs /
   RecapReport (src/types.ts) carrying each login's merged-PR count for the scan window, computed by
   tallying listRecentMergedPullRequests results per repo (filtered to mergedAt within the recap
   windowDays), mirroring buildMaintainerQualityDashboard's contributorTotals Map-tally pattern
   (src/services/maintainer-quality-dashboard.ts lines 93, 142-153) — group by authorLogin ?? "unknown",
   sum counts across every scan repo (a contributor active in multiple repos gets one combined total, not one
   row per repo).
2. buildMaintainerRecap (src/services/maintainer-recap.ts) must fold this contributor data into the
   RecapReport it returns.
3. runMaintainerRecapJob (src/review/maintainer-recap-wire.ts) must gather the per-repo merged-PR data
   (via listRecentMergedPullRequests) inside its existing per-repo loop, with the same per-repo
   try/catch-and-skip fail-safe already used for the other two aggregators in that loop — a D1 hiccup on one
   repo must not blank the whole digest, exactly like every other aggregator load in that function.
4. formatMaintainerRecap (src/services/maintainer-recap.ts) must call buildTopContributorsRecapSection
   and render its ## Top contributors section into the digest, unconditionally (not behind an
   options flag) — same treatment fix(notifications): wire the maintainer-recap's calibration/gate-outcomes/per-repo section builders into the actual digest output #8372 gave calibrationSection/gateOutcomesSection, since the input
   data is always present on every RecapReport once Requirement 1 lands. Route the section's title/lines
   through the existing redactRecapLine/recapSectionLines helpers exactly like the other unconditional
   sections do.
5. An empty/zero-contributor window must render the section's existing empty-state fallback line, not omit
   the heading — mirror the recapSectionLines(..., "_No … for this window._") convention already used for
   every other section in formatMaintainerRecap.
6. No change to what data is publicly emitted beyond what buildTopContributorsRecapSection itself already
   redacts (it already gates every line through isPublicSafeText — do not weaken or duplicate that gate).

Deliverables

• RecapReport/MaintainerRecapInputs (src/types.ts, src/services/maintainer-recap.ts) carry a
  per-login merged-PR contributor tally for the window.
• runMaintainerRecapJob (src/review/maintainer-recap-wire.ts) populates that tally from
  listRecentMergedPullRequests, window-filtered by mergedAt, with the same per-repo fail-safe
  try/catch pattern as its sibling aggregator loads in the same loop.
• formatMaintainerRecap unconditionally renders buildTopContributorsRecapSection's output as a
  ## Top contributors section, using the existing redactRecapLine/recapSectionLines helpers.
• A unit test in test/unit/maintainer-recap-format.test.ts (or maintainer-recap.test.ts) asserting a
  formatMaintainerRecap output now DOES contain ## Top contributors with the expected ranked
  login/merged lines for a report with contributor data, and DOES contain the section's empty-state
  fallback line when the window has zero contributors.
• A unit test in test/unit/maintainer-recap-wire.test.ts (or equivalent) asserting
  runMaintainerRecapJob/buildMaintainerRecap correctly aggregates merged-PR counts by login across
  multiple repos in the window, and that a per-repo listRecentMergedPullRequests failure is
  swallowed/skipped rather than aborting the whole digest (mirroring the existing
  maintainer_recap_repo_error fail-safe test coverage for the other two aggregators).

All deliverables above are required in this one PR — there is no narrower intentionally-deferred scope.

Test Coverage Requirements

99%+ Codecov patch coverage (branch-counted) on every changed/added line in src/**, measured unsharded via
npm run test:coverage. This is a bug fix (dead code that was supposed to be live output, same class as
#8372/#6636) — include a regression test that fails against the pre-fix behavior (asserting ## Top contributors is present in the rendered digest) and passes after the fix.

Expected Outcome

The maintainer-recap digest delivered to Discord/Slack now includes a "Top contributors" section (ranked
merged-PR counts by login, public-safe, capped and empty-state-handled) for every run that has contributor
data — closing the same "built, tested, never shipped" gap #8372 already closed for this section's four
siblings.

Links & Resources

• Precedent to mirror exactly: fix(notifications): wire the maintainer-recap's calibration/gate-outcomes/per-repo section builders into the actual digest output #8372 (closed) — fixed the identical bug shape for
  buildCalibrationRecapSection / buildGateOutcomesRecapSection / buildPerRepoRecapSection /
  buildDriftRecapSection.
• src/services/maintainer-recap-top-contributors.ts — the already-implemented, already-tested, never-wired
  section builder (buildTopContributorsRecapSection).
• src/services/maintainer-recap.ts — buildMaintainerRecap / formatMaintainerRecap (the composition
  points to extend).
• src/review/maintainer-recap-wire.ts — runMaintainerRecapJob (the per-repo data-loading loop to extend).
• src/services/maintainer-quality-dashboard.ts lines 93, 142-160 — the contributorTotals per-login
  Map-tally-sort-cap pattern to mirror.
• src/db/repositories.ts line 5090 (listRecentMergedPullRequests) and src/types.ts
  (RecentMergedPullRequestRecord, authorLogin/mergedAt) — the existing data source, no new query needed.
• test/unit/maintainer-recap-top-contributors.test.ts — existing coverage for the section builder itself
  (unaffected by this change).

[linear-code]

JSO-6447