refactor(review): simplify pass over E (reuse, simplification, efficiency)

Quality only, no behaviour change (four cleanup reviewers, one implementer;
npm test 2480/0 on the working tree):

- threat-model.mjs splitBacked(): the one known_outside backing check of
  plan-gate (record and bare gate), verdict-gate and review-finalize, with
  one why-text; the engines' security keys derived from ENGINES; isSecurity.
- coordinator-contract: "not a security engine" read from ENGINES (a string
  engine only), not the code-review literal.
- review-rounds owns trailingBlock, findingsBlockSha, isMajor, isBlocker;
  review-answers imports them (review-rounds still imports nothing from it).
  prepare keeps the archive bytes it read instead of reading the previous
  archive again.
- engine-compare exports rowIn and payloadEngine; finalize uses them.
- plan-gate: one accepted_open builder; the stop-rule check hoisted; one
  top-level lookup for --record --brief.
- security-engine: notFullShas(), runCli's usage flag, readScopeList shared
  with engine-findings; findReport and reviewRoundSummary module-private;
  two dead defaults gone.
- tests: shared plan-gate harness and threat-model brief helpers; the [CNV]
  table cases call convertReport in-process, one CLI case per exit path.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/helpers/plan-gate-harness.mjs

@@ -0,0 +1,81 @@
+// tests/helpers/plan-gate-harness.mjs
+// The critic-round and CLI harness of the plan-gate tests
+// (tests/validators/plan-gate-diff-mode, -stop-rule and -threat-model):
+// rounds through the real recordVerdict and dedup on buildPlan() text, and
+// plan-gate.mjs run as a child in its own temp `git init` repo.
+
+import { strict as assert } from 'node:assert';
+import { spawnSync } from 'node:child_process';
+import { mkdtempSync, realpathSync, rmSync } from 'node:fs';
+import { tmpdir } from 'node:os';
+import { dirname, join, resolve } from 'node:path';
+import { fileURLToPath } from 'node:url';
+import { recordVerdict, planHash, operativeText } from '../../lib/validators/plan-gate.mjs';
+import { dedupFindings } from '../../lib/review/plan-review-dedup.mjs';
+import { childEnv } from './child-env.mjs';
+
+export const ROOT = join(dirname(fileURLToPath(import.meta.url)), '..', '..');
+export const PATHS = [resolve('plan.md')];
+
+/** The 1-based full-file line holding `s` (findIndex over the whole text). */
+export function at(text, s) {
+  const n = text.split('\n').findIndex(l => l.includes(s)) + 1;
+  assert.ok(n > 0, `the plan holds ${JSON.stringify(s)}`);
+  return n;
+}
+
+/** The base a round diffs against: the operative text the previous round reviewed. */
+export const baseOf = t => ({ sha256: planHash(t), text: operativeText(t) });
+
+/** `rec(text, d, opts)`: one round as Phase 9 runs it, recorded at `now`, with the plan path known to the gate. */
+export function recorder(now) {
+  return (text, d, opts = {}) => {
+    const r = recordVerdict(text, d, { reviewed: planHash(text), planPaths: PATHS, now, ...opts });
+    assert.equal(r.code, 0, r.message);
+    return r;
+  };
+}
+
+/** Reviewer payloads through the real dedup. */
+export function dd({ pc = [], sg = [], pcPrev, sgPrev } = {}) {
+  return dedupFindings([
+    { agent: 'plan-critic', payload: { findings: pc, ...(pcPrev ? { previous: pcPrev } : {}) } },
+    { agent: 'scope-guardian', payload: { findings: sg, ...(sgPrev ? { previous: sgPrev } : {}) } },
+  ]);
+}
+
+/** A reviewer finding on plan.md under `rule_key`. */
+export const keyedFinding = (severity, line, text, rule_key, extra = {}) => ({ file: 'plan.md', line, rule_key, severity, text, ...extra });
+
+export const R1_CHANGES = 'Add `run1()`';
+export const R2_CHANGES = '- **Changes:** Add `run2()` to `src/mod2.mjs`. (new file)';
+export const R2_EDITED = '- **Changes:** Add `run2()` and its error path to `src/mod2.mjs`. (new file)\n  - `run2()` throws on empty input.';
+
+/** Edit A: Step 2's Changes line becomes two lines (a two-line hunk, nothing above it moves). */
+export function editA(t) {
+  assert.ok(t.includes(R2_CHANGES));
+  return t.replace(R2_CHANGES, R2_EDITED);
+}
+
+// ---- CLI ------------------------------------------------------------------------
+
+export const GATE = join(ROOT, 'lib', 'validators', 'plan-gate.mjs');
+export const GIT_CLEAN = { GIT_DIR: undefined, GIT_WORK_TREE: undefined, GIT_INDEX_FILE: undefined };
+
+/** plan-gate.mjs as a child in `cwd`, with `input` on stdin. */
+export function runGate(cwd, args, input) {
+  const r = spawnSync(process.execPath, [GATE, ...args], { cwd, input, encoding: 'utf-8', env: childEnv(GIT_CLEAN) });
+  return { code: r.status, out: r.stdout, err: r.stderr };
+}
+
+/** The last non-empty output line, parsed. */
+export const lastJson = out => JSON.parse(out.split('\n').filter(l => l !== '').at(-1));
+
+/** A fresh temp dir (real path), `git init`-ed; removed again when the init fails. */
+export function tempRepo(prefix) {
+  const dir = realpathSync(mkdtempSync(join(tmpdir(), prefix)));
+  const g = spawnSync('git', ['init', '-q'], { cwd: dir, encoding: 'utf-8', env: childEnv(GIT_CLEAN) });
+  if (g.status !== 0) rmSync(dir, { recursive: true, force: true });
+  assert.equal(g.status, 0, g.stderr);
+  return dir;
+}

tests/helpers/threat-model-brief.mjs

@@ -0,0 +1,41 @@
+// tests/helpers/threat-model-brief.mjs
+// The brief fixture of the threat-model tests (tests/lib/threat-model and
+// tests/validators/brief-threat-model): a brief_version 2.2 brief with an
+// optional `## Threat Model` section before its Success Criteria.
+
+export const OUTSIDE = "a local user who can already edit the repo's hooks";
+export const INSIDE = 'a crafted file name in a pull request reaches the shell';
+export const TM = `## Threat Model\n\n- Inside: ${INSIDE}\n- Outside: ${OUTSIDE}\n`;
+
+/** The brief with `threat` (a `## Threat Model` section, or '') before its Success Criteria, of frontmatter `type`. */
+export const briefWith = (threat, type = 'trekbrief') => `---
+type: ${type}
+brief_version: "2.2"
+created: 2026-11-08
+task: "Add an upload hook"
+slug: upload-hook
+project_dir: .claude/projects/2026-11-08-upload-hook/
+research_topics: 0
+research_status: complete
+framing: new-direction
+---
+
+# Task
+
+## TL;DR
+
+An upload hook.
+
+## Intent
+
+Uploads arrive from strangers.
+
+## Goal
+
+A hook that checks each upload.
+
+${threat}
+## Success Criteria
+
+- It checks.
+`;