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.js

@@ -0,0 +1,34 @@
+// Pure, side-effect-free helpers for the resolver. Kept separate from resolver.js
+// (which has the HTTP-server side effect) so they can be unit-tested with `node --test`
+// without starting a server. The artist-match precision lever lives here.
+
+export const norm = (s) => String(s).normalize("NFKC").trim().toLowerCase();
+
+export const bandcampSearch = (n) => `https://bandcamp.com/search?q=${encodeURIComponent(n)}&item_type=b`;
+export const spotifySearch = (n) => `https://open.spotify.com/search/${encodeURIComponent(n)}`;
+
+// The precision lever: return the result whose name matches the candidate
+// case-insensitively, else null. `minScore` (MusicBrainz) additionally requires a
+// confidence score. Tolerates null/garbage items.
+export function pickArtist(name, items, { minScore = null } = {}) {
+  const want = norm(name);
+  return (
+    (items ?? []).find(
+      (a) => a && norm(a.name) === want && (minScore == null || (a.score ?? 0) >= minScore),
+    ) ?? null
+  );
+}
+
+// Link object for a confirmed Spotify hit (direct artist url when present, else search).
+export const spotifyResult = (hit) => ({
+  name: hit.name,
+  spotify: hit.external_urls?.spotify ?? spotifySearch(hit.name),
+  bandcamp: bandcampSearch(hit.name),
+});
+
+// Link object for a confirmed MusicBrainz hit (search urls — MB gives no platform ids).
+export const searchResult = (hit) => ({
+  name: hit.name,
+  spotify: spotifySearch(hit.name),
+  bandcamp: bandcampSearch(hit.name),
+});