feat(reviewers): per-reviewer throughput and turnaround report

[boskodev790]

what

vouch reviewers — a read-only throughput report keyed on the human at the
gate.

vouch stats buckets the review queue by the proposing agent (proposed_by)
and vouch digest lists recent decisions one at a time. neither answers "who
is doing the reviewing, and how" — the decided_by axis. this reports, per
reviewer:

• decisions, split into approved vs rejected, and the approval rate
• turnaround — how long proposals waited from proposed_at to decided_at,
  as median and max

an auto-expiry is not a review decision — a proposal that ages out is closed by
the vouch-expire actor, not a human — so expiries are excluded from every
reviewer's counts and reported once, separately, as expired_total (same
EXPIRE_REASON convention stats uses). clock-skew decisions (decided before
proposed) still count but contribute no turnaround sample.

why

on a team with more than one human at the gate, this is the "who's carrying the
review load, who's the bottleneck, who rubber-stamps" view — none of which the
by-proposer surfaces show. safe from cron via --since / --format json.

surface

strictly a viewport. composes store.list_proposals; writes nothing, logs no
audit event, never touches a proposal. cli-only, so no new kb.* method (the
four-site method surface and test_capabilities are untouched). no storage,
model, migration, or on-disk-layout change.

• src/vouch/reviewers.py — new module: pure build() plus text / markdown /
  json renderers with a stable to_dict() json contract
• src/vouch/cli.py — vouch reviewers, next to vouch digest
• tests/test_reviewers.py — 10 tests (per-reviewer counts + approval rate,
  expiry exclusion, turnaround, --since window, unknown-reviewer fallback,
  clock-skew drop, and the cli)
• CHANGELOG.md — [Unreleased]

example

reviewer throughput @ 2026-07-10T09:00:00+00:00  (window since: 30d)
reviewers: 2  decisions: 34  expired (excluded): 3

  alice  22 decision(s)  19 approved / 3 rejected  approval 86%  turnaround med 2.4h / max 41h
  bob    12 decision(s)  8 approved / 4 rejected  approval 67%  turnaround med 6.1h / max 88h

validation

• python -m pytest tests/ -q --ignore=tests/embeddings → 1098 passed, 37 skipped
• python -m mypy src → clean
• python -m ruff check src tests → clean

no linked issue — a self-contained read-only viewport that changes neither the
object model, the kb.* surface, the on-disk layout, the bundle format, nor
the audit shape, so per contributing it doesn't need a vep. glad to file one if
you'd prefer to track it.

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