docs(review): document gate.copycat.mode and fix docs-drift check

The scaffold PR added copycatGateMode to RepositorySettings but missed
two things docs:drift-check enforces for every *GateMode field: a
GATE_MODE_MANIFEST entry in scripts/check-docs-drift.mjs, and real
documentation on docs.how-reviews-work.tsx + docs.tuning.tsx. Both added,
honestly noting the detection engine hasn't shipped yet. Bumps the
synthetic-fixture gate-mode count in the drift-check's own test to match.

Author: JSONbored

apps/gittensory-ui/src/routes/docs.how-reviews-work.tsx

@@ -109,6 +109,14 @@ function HowReviewsWork() {
           warnings; <code>block</code> also hard-blocks at or above <code>slopGateMinScore</code>{" "}
           (engine default band <code>60</code>).
         </li>
+        <li>
+          <strong>Copycat / plagiarism gate</strong> (<code>copycatGateMode</code>, default{" "}
+          <code>off</code>) — a code containment/similarity check against prior art (repo history,
+          other PRs). Escalating tiers: <code>warn</code>, <code>label</code>,{" "}
+          <code>block</code>, plus a further strikes escalation for repeat offenders. Config only
+          today — the detection engine itself has not shipped yet, so setting this has no effect
+          until it does.
+        </li>
         <li>
           <strong>Merge-readiness gate</strong> (<code>mergeReadinessGateMode</code>, default{" "}
           <code>off</code>) — a composite readiness check.

apps/gittensory-ui/src/routes/docs.tuning.tsx

@@ -303,6 +303,14 @@ function Tuning() {
           a free advisory-only <code>ai_slop_advisory</code> finding — it never feeds the slop score
           or the gate.
         </li>
+        <li>
+          <code>gate.copycat.mode</code> — code containment/similarity gate against prior art
+          (repo history, other PRs). Default <code>off</code>. Escalating tiers:{" "}
+          <code>warn</code>, <code>label</code>, <code>block</code>, plus a further strikes
+          escalation for repeat offenders. Pair it with <code>gate.copycat.minScore</code>{" "}
+          (0–100). Config only today — the detection engine has not shipped yet, so this has no
+          effect until it does.
+        </li>
         <li>
           <code>gate.mergeReadiness</code> — composite merge-readiness gate. Default{" "}
           <code>off</code>, no min score.

scripts/check-docs-drift.mjs

@@ -53,6 +53,7 @@ export const GATE_MODE_MANIFEST = [
   { field: "duplicatePrGateMode", aliases: ["duplicatePrGateMode", "gate.duplicates"], pages: ["docs.how-reviews-work.tsx", "docs.tuning.tsx"] },
   { field: "qualityGateMode", aliases: ["qualityGateMode", "gate.readiness.mode"], pages: ["docs.how-reviews-work.tsx", "docs.tuning.tsx"] },
   { field: "slopGateMode", aliases: ["slopGateMode", "gate.slop.mode"], pages: ["docs.how-reviews-work.tsx", "docs.tuning.tsx"] },
+  { field: "copycatGateMode", aliases: ["copycatGateMode", "gate.copycat.mode"], pages: ["docs.how-reviews-work.tsx", "docs.tuning.tsx"] },
   { field: "sizeGateMode", aliases: ["sizeGateMode", "gate.size"], pages: ["docs.how-reviews-work.tsx", "docs.tuning.tsx", "docs.github-app.tsx"] },
   { field: "lockfileIntegrityGateMode", aliases: ["lockfileIntegrityGateMode", "gate.lockfileIntegrity"], pages: ["docs.how-reviews-work.tsx", "docs.tuning.tsx", "docs.github-app.tsx"] },
   { field: "claGateMode", aliases: ["claGateMode", "gate.claMode"], pages: ["docs.how-reviews-work.tsx", "docs.tuning.tsx", "docs.github-app.tsx"] },

test/unit/check-docs-drift-script.test.ts

@@ -141,8 +141,8 @@ describe("check-docs-drift script", () => {
       const result = checkDocsDrift({ root: "/fake", readFile: makeReadFile(files) });
 
       expect(result.failures).toEqual([]);
-      // gateModes bumped 11 -> 12 for linkedIssueSatisfactionGateMode (#1961/#3906).
-      expect(result.counts).toEqual({ flags: 10, commands: 19, gateModes: 12 });
+      // gateModes bumped 12 -> 13 for copycatGateMode (#1969, currently inert config scaffold).
+      expect(result.counts).toEqual({ flags: 10, commands: 19, gateModes: 13 });
     });
 
     it("catches an unmapped *GateMode field missing from GATE_MODE_MANIFEST", () => {