Migrate CI setup to pnpm/setup #1119

.github/actions/setup/action.yml pins three things to get a build environment: pnpm/action-setup for pnpm, actions/setup-node for Node 22, and a pnpm install --frozen-lockfile step. pnpm's continuous integration docs now recommend a single action instead:

- uses: pnpm/setup@<sha> # v2.1.0
  with:
    runtime: node@22
    cache: true

pnpm/setup installs pnpm, installs the runtime with pnpm runtime set and puts it on PATH (replacing actions/setup-node), caches the pnpm store, and runs pnpm install unless you pass install: false. Two pinned actions and an install step collapse into one.

Check before switching

• Store cache. fix(ssh): repair managed Windows SSH config permissions #1110 moved the store cache to pnpm/action-setup's cache: true, which keys on pnpm-cache-<os>-<arch>-<lockfile hash> and falls back to the last store for the platform, so a dependency bump fetches only what changed. Confirm pnpm/setup keeps that fallback; without it every bump refetches the whole store.
• Frozen installs. Its automatic pnpm install must stay frozen in CI, as --frozen-lockfile does today. require-lockfile looks related.
• Windows on ARM64. The windows-11-arm job needs pnpm runtime set to fetch a win32-arm64 Node build.
• Node version. CI runs Node 22 to match VS Code's Electron, and engines.node is >= 22. runtime: node@22 has to hold that, and CONTRIBUTING's list of files to update when the minimum moves needs this one added.

Baseline

Times from the run on #1110, for comparison after the switch:

Pin by SHA, like the other actions. Latest release is v2.1.0.