docs(contributing): add reviewer-ladder.md — concrete proposal for the reviewer-scaling rung

CONTRIBUTING.md

@@ -58,3 +58,6 @@ Full layer validation (the `common` behave suite from
 - [`docs/contributing/style-guide.md`](docs/contributing/style-guide.md) —
   coding and configuration conventions for shell scripts, Just recipes,
   JSON/YAML, and the Containerfile.
+- [`docs/contributing/reviewer-ladder.md`](docs/contributing/reviewer-ladder.md) —
+  draft proposal for a four-rung contributor ladder (Triager, Domain
+  Reviewer); unadopted until a maintainer decision.

docs/contributing/reviewer-ladder.md

@@ -0,0 +1,108 @@
+# Reviewer Ladder — Contribution Strategy (Planning Draft)
+
+> **Status**: hold-gated planning artifact. Human review required before adoption.
+> Filed by the strategist agent. Tracks [common#1029](https://github.com/projectbluefin/common/issues/1029);
+> Proposed alongside the org-wide roadmap in [common#1073](https://github.com/projectbluefin/common/pull/1073) (landed as ROADMAP.md; its phases remain proposals).
+>
+> Nothing below changes GitHub roles, branch protection, CODEOWNERS, workflow behavior,
+> or hold-gate enforcement until a maintainer adopts it.
+
+## Problem
+
+Reviewer coverage is the binding constraint in several repos, and the contributor
+ladder has no rung between "opens PRs" and "maintainer" that grows it. Org-wide
+throughput is healthy, but reviewer coverage is uneven across repos.
+
+Reported state, using snapshots dated 2026-08-27 through 2026-09-05:
+
+- **88 open hold-gated agent PRs** across 10 repos compete for limited
+  per-repo reviewer coverage ([common#1043](https://github.com/projectbluefin/common/issues/1043): no
+  prioritization rubric; the repo-concentration breakdown is in [common#1058](https://github.com/projectbluefin/common/issues/1058)).
+- The lowest throughput is concentrated in common (4 merges), server (1),
+  actions (0), and dakota-iso (0) while other repos drain ([common#1058](https://github.com/projectbluefin/common/issues/1058),
+  last-7-day snapshot).
+- Human-filed issues starve alongside: 74 issues in `1-triage`, 32 aged >60
+  days ([common#1029](https://github.com/projectbluefin/common/issues/1029),
+  2026-08-27 snapshot); 50 human-authored issues in triage, 17 aged >90
+  days ([common#1067](https://github.com/projectbluefin/common/issues/1067),
+  2026-08-31 snapshot).
+- Downstream release impact at the 2026-09-04 snapshot: bluefin stable frozen
+  46 days (stable-20260720), bluefin-lts 28 days stale, common missed its
+  v2026.09 monthly tag, while dakota stable was current (0 days old)
+  ([common#1078](https://github.com/projectbluefin/common/issues/1078)).
+
+The roadmap proposed in [common#1073](https://github.com/projectbluefin/common/pull/1073)
+places "reviewer-scaling rung" in Phase 0. This document is the concrete
+proposal for that rung; both are pending maintainer decision, and this text
+stands on its own if the roadmap is reshaped.
+
+## Proposal: a four-rung ladder
+
+| Rung | Role | Scope | Proposed grants |
+|------|------|-------|--------|
+| 0 | Contributor | any | fork PRs |
+| 1 | **Triager** | per-repo | label, dedupe, reproduce, close-as-duplicate |
+| 2 | **Domain Reviewer** | per-repo, per-domain | approving review on hold-gated PRs within an owned domain (e.g. justfiles, CI workflows, tests); cannot merge |
+| 3 | Maintainer | per-repo | merge, release |
+
+All permissions in this table are proposals, not current repository policy.
+
+Rungs 1–2 are the new rungs. Both are **additive permissions**, reachable
+without full maintainer trust, and directly attack the two measured
+bottlenecks: triage age (rung 1) and hold-gate depth (rung 2).
+
+### Promotion criteria (observable, no nomination-by-vibes)
+
+- **Contributor → Triager** (proposed): 5+ merged PRs in the repo **or** 10
+  substantive triage actions (reproduction notes, dedupe links, label
+  corrections) on others' issues. A maintainer would confirm; no vote.
+- **Triager → Domain Reviewer** (proposed): 30 days as Triager **and** 10
+  reviewed hold-gated PRs in the claimed domain with maintainer sign-off on
+  review quality. The domain would be recorded in the repo's contributor doc.
+- **Domain Reviewer → Maintainer**: existing org process; unchanged.
+
+### Why domain-scoped review
+
+[common#1058](https://github.com/projectbluefin/common/issues/1058) shows
+throughput is uneven across specific repos, not uniformly low. Domain-scoped
+reviewers could let a trusted contributor unblock, say, `justfiles` in common
+or `workflows` in dakota-iso without granting org-wide merge rights — matching
+permission scope to the measured bottleneck.
+
+### Interaction with the hold-gate
+
+Agent-filed hold-gated PRs currently require human review before merge. Under
+this draft, a Domain Reviewer's approval would satisfy the review requirement,
+while a maintainer would still merge. This is a proposal only: it does not
+change current permissions, branch protection, CODEOWNERS, or workflow
+enforcement.
+
+## Operating metrics
+
+If the roadmap proposed in [common#1073](https://github.com/projectbluefin/common/pull/1073)
+is adopted, its success table can track adoption; either way these are
+draft operating measures:
+
+- Open hold-gated PR count (baseline: 88 on 2026-09-05) — ROADMAP target: <40
+  and shrinking
+- Median hold-gated PR review latency — draft target: <7 days
+- Human-issue triage age — ROADMAP baseline: 50+ issues >30d on 2026-09-02,
+  target: 0; supporting snapshots report 32 issues >60d on 2026-08-27 and
+  17 >90d on 2026-08-31
+- Zero-throughput repos ([common#1058](https://github.com/projectbluefin/common/issues/1058))
+  — draft target: ≥1 merged agent PR per repo/month
+
+## What this document deliberately does not do
+
+- No workflow/CI changes (ci-maintainer lane).
+- No changes to merge protection or CODEOWNERS (maintainer decision).
+- No agent-lane throttling mechanics (tracked separately in [common#1052](https://github.com/projectbluefin/common/issues/1052)).
+
+## Related
+
+- [common#1029](https://github.com/projectbluefin/common/issues/1029) — reviewer-scaling rung finding (this doc's tracker)
+- [common#1073](https://github.com/projectbluefin/common/pull/1073) — org-wide ROADMAP.md (Phase 0 parent)
+- [common#1058](https://github.com/projectbluefin/common/issues/1058) — reviewer coverage map / repo-concentration data
+- [common#1067](https://github.com/projectbluefin/common/issues/1067) — user-issue triage starvation
+- [common#1043](https://github.com/projectbluefin/common/issues/1043) — hold-gate prioritization rubric
+- [common#1078](https://github.com/projectbluefin/common/issues/1078) — release cadence breakdown (downstream symptom)