Summary

Upgrades tooling to Yarn 4 (Berry) and adds a Node 22 + 24 CI test matrix, bringing
x401-node in line with proof-vc-common. Also refreshes Dependabot config and the
Node-version docs.

Changes

Yarn 1 → 4.17.0 (Berry)

• Pin Yarn via packageManager: [email protected] + .yarn/releases/yarn-4.17.0.cjs; run
  through Corepack.
• Add .yarnrc.yml: nodeLinker: node-modules, enableImmutableInstalls,
  enableScripts: false (no postinstall scripts), npmMinimalAgeGate: 1w,
  enableGlobalCache.
• Regenerate yarn.lock in the Berry format.
• publish.yml: add corepack enable, drop --frozen-lockfile (immutable installs
  are the default now).
• .gitignore / .prettierignore updated for the Berry layout (keep
  .yarn/releases etc., ignore .yarn/* and spec/).
• Rename eslint.config.js → eslint.config.mjs.
• Bump devDeps: @types/node ^25.9.1 → ^24.13.2, eslint ^10.4.0 → ^10.5.0,
  typescript-eslint ^8.61.0 → ^8.62.0.

CI: Node 22 + 24 test matrix

• Split the test job into test-matrix (runs yarn test on Node 22 and 24) and an
  aggregate test gate (needs: test-matrix, if: always()).
• The gate keeps a single stable required check named test for the branch ruleset,
  so the matrix legs (test (22) / test (24)) don't need to be enumerated in branch
  protection. always() makes a cancelled leg fail the gate rather than pass silently.
• Other jobs (format, lint, typecheck, build, publint) continue on Node 24 from
  .node-version.

Dependabot

• Add a 7-day cooldown (matches npmMinimalAgeGate).
• Ignore @types/node major bumps so it tracks the active LTS (24), not latest.

Docs

• CLAUDE.md Rule 7: engines.node >=22 is the consumer floor (dist uses only
  long-stable globals); dev/CI run on Node 24; the CI matrix now verifies the floor at
  runtime. Note that the Node 22 leg resolves to the latest 22.x because the native
  .ts test runner needs default type stripping (Node >=22.18) — don't pin the low leg
  below that.
• CONTRIBUTING.md: fix the requirement (was >=24, now >=22 published floor with
  dev on 24) and document the CI matrix.

Rationale

engines.node stays at >=22.0.0 — the consumer floor, since the shipped dist only
uses long-stable globals and runs on any maintained LTS. @types/node tracks the dev /
active-LTS line (24); the runtime floor is now guarded by the CI matrix rather than the
type version.

Test plan

• CI: format / lint / typecheck / build / publint on Node 24; test on Node 22 and 24.
• Local: yarn test → 32 pass; yarn build && yarn publint → clean.