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>

scripts/check.sh

@@ -0,0 +1,20 @@
+#!/usr/bin/env bash
+# Local check gate for the linker (owned by the quality steward — plan/steward-linker-quality).
+# Zero-dep: only `node` is required. Run before every push.
+set -euo pipefail
+cd "$(dirname "$0")/.."
+
+echo "== node --check (syntax) =="
+for f in $(find service extension -name '*.js'); do
+  node --check "$f"
+  echo "  ok $f"
+done
+
+echo "== JSON validity =="
+node -e "JSON.parse(require('fs').readFileSync('extension/manifest.json','utf8')); JSON.parse(require('fs').readFileSync('service/package.json','utf8'))"
+echo "  ok extension/manifest.json + service/package.json"
+
+echo "== node --test (unit) =="
+( cd service && node --test )
+
+echo "ALL CHECKS PASSED"