feat: stewardship doctrine + zero-dep test infra (closes issue/linker-code-stewards-and-test-infra)

Stand up code stewards + testing for the linker, adapted to its JS/Node stack:
- service/lib.js: extract the resolver's pure logic (norm + exact-name match + link
  builders) so it is unit-testable without the server side effect; resolver.js imports it.
- service/lib.test.js: node:test coverage of the precision lever (7 tests).
- scripts/check.sh: zero-dep gate (node --check + JSON validation + node --test).
- AGENTS.md: repo doctrine — plain-repo push flow (NOT towl's PR pipeline), the three
  stewards (quality/design/security), the test gate, standing decisions (MV2 stays; no ML).

Source: issue/linker-code-stewards-and-test-infra (inbox human-feedback 2026-06-16 —
"should have a set of code stewards ... ensure a testing infra gets setup and maintained").

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

service/lib.test.js

@@ -0,0 +1,51 @@
+// Unit tests for the resolver's pure logic (run with `node --test`). Zero-dep.
+import { test } from "node:test";
+import assert from "node:assert/strict";
+import { norm, bandcampSearch, spotifySearch, pickArtist, spotifyResult, searchResult } from "./lib.js";
+
+test("norm lowercases, trims, NFKC-normalizes", () => {
+  assert.equal(norm("  Cerebral Bore "), "cerebral bore");
+  assert.equal(norm("DEVOURMENT"), "devourment");
+});
+
+test("pickArtist matches case-insensitively, rejects non-matches", () => {
+  const items = [{ name: "Cattle Decapitation" }, { name: "Cerebral Bore" }];
+  assert.equal(pickArtist("cerebral bore", items)?.name, "Cerebral Bore");
+  assert.equal(pickArtist("CATTLE DECAPITATION", items)?.name, "Cattle Decapitation");
+  assert.equal(pickArtist("Notarealband", items), null);
+});
+
+test("pickArtist enforces minScore when given (MusicBrainz path)", () => {
+  const items = [{ name: "Devourment", score: 100 }, { name: "Devourment", score: 50 }];
+  assert.equal(pickArtist("Devourment", items, { minScore: 90 })?.score, 100);
+  assert.equal(pickArtist("Devourment", [{ name: "Devourment", score: 50 }], { minScore: 90 }), null);
+});
+
+test("pickArtist tolerates empty / garbage input", () => {
+  assert.equal(pickArtist("x", null), null);
+  assert.equal(pickArtist("x", []), null);
+  assert.equal(pickArtist("x", [null, { name: "y" }]), null);
+});
+
+test("spotifyResult prefers the direct url, falls back to search", () => {
+  const direct = spotifyResult({
+    name: "Devourment",
+    external_urls: { spotify: "https://open.spotify.com/artist/abc" },
+  });
+  assert.equal(direct.spotify, "https://open.spotify.com/artist/abc");
+  assert.match(direct.bandcamp, /^https:\/\/bandcamp\.com\/search\?q=Devourment/);
+
+  const noUrl = spotifyResult({ name: "Devourment" });
+  assert.equal(noUrl.spotify, "https://open.spotify.com/search/Devourment");
+});
+
+test("searchResult builds spotify + bandcamp search urls", () => {
+  const r = searchResult({ name: "Cerebral Bore" });
+  assert.equal(r.spotify, "https://open.spotify.com/search/Cerebral%20Bore");
+  assert.equal(r.bandcamp, "https://bandcamp.com/search?q=Cerebral%20Bore&item_type=b");
+});
+
+test("link builders url-encode", () => {
+  assert.equal(spotifySearch("A B"), "https://open.spotify.com/search/A%20B");
+  assert.ok(bandcampSearch("A&B").includes("A%26B"));
+});