build: migrate from yarn to pnpm

[AliceR]

Stacked on #68 (dependency updates). Merge that first; GitHub will then retarget this PR to main.

Migrates the library and the example app from Yarn 4 to pnpm 12.

Changes

• Lockfiles: yarn.lock → pnpm-lock.yaml via pnpm import, so resolved versions are unchanged. Done for both the root and example/.
• packageManager: pnpm@12.8.1. .yarnrc.yml removed.
• Install scripts: pnpm blocks dependency build scripts by default. Explicitly denied in pnpm-workspace.yaml (allowBuilds):
  • @parcel/watcher, unrs-resolver (via Jest): their scripts are only fallbacks; the prebuilt binaries arrive as optional deps
  • core-js, core-js-pure (example): postinstall banner only
• Example app: has its own pnpm-workspace.yaml, so it stays a standalone project. The link: setup for react, react-dom and @tanstack/react-query is unchanged; verified that the app and the library resolve a single copy of each.
• CI: uses pnpm/action-setup (v6.1.0, SHA-pinned) with its pnpm-store cache keyed on pnpm-lock.yaml. This replaces corepack and the node_modules cache keyed on package.json. Corepack cannot run pnpm 12 (it expects bin/pnpm.cjs, but pnpm 12 ships native binaries), and Node 25+ no longer bundles corepack.
• CI fix: the build job was running yarn test instead of the build. It now runs pnpm build.
• Docs: README install instructions add pnpm; dev commands now use pnpm.

Fixes surfaced by pnpm's strict node_modules

• fix: published types referenced @tanstack/query-core, which is not a dependency of this package. dist/src/context/index.d.ts contained import('@tanstack/query-core').QueryClient, which fails type checking for consumers whose package manager doesn't hoist query-core (e.g. pnpm without skipLibCheck). Now references @tanstack/react-query, which re-exports it.
• queryKeys.test.ts imported the undeclared @jest/globals; it now uses the Jest globals like the other suites.

Local setup

Install pnpm 12 (https://pnpm.io/installation), for example npm i -g pnpm, then run pnpm install. Note that corepack enable won't work with pnpm 12.

Verification

From a clean state: pnpm install --frozen-lockfile, pnpm lint, pnpm test (94/94) and pnpm build all pass. pnpm pack --dry-run contents match the previous build. The example app installs and pnpm build compiles.

Not in this PR

• StacApiProvider.test.d.ts is published because the dts exclude only matches *.test.ts. This predates the migration.
• Dependabot only tracks GitHub Actions; an npm ecosystem entry (works for pnpm) could be added.

🤖 Generated with Claude Code