ci: add PR pipeline (pnpm via corepack, turbo typecheck, tests, build)

[obvious-autobuild]

Why

PRs to master currently have no automated verification — nothing runs install, typecheck, tests, or build before merge. This PR adds the CI pipeline for the agreed monorepo scaffold: pnpm (via corepack) on Node 20, turbo-orchestrated typecheck, unit tests, and build, triggered on every pull request to master.

What

.github/workflows/ci.yml — single job on ubuntu-latest:

1. Checkout + Node 20 (actions/checkout@v4, actions/setup-node@v4)
2. pnpm via corepack — corepack enable; the scaffold's root packageManager field will pin the exact pnpm version once it lands (corepack falls back to its default until then)
3. pnpm store cache — keyed on the pnpm-lock.yaml hash (official pnpm caching recipe), so installs warm up after the scaffold lands
4. pnpm install --frozen-lockfile — CI never mutates the lockfile
5. pnpm turbo run typecheck
6. pnpm turbo run test — meaningful unit tests live in the domain/extraction packages per the agreed structure; running the turbo test task covers every package defining tests without hardcoding package names
7. pnpm turbo run build

Extras: permissions: contents: read (least privilege), a concurrency group that cancels superseded runs on the same PR, and a 20-minute job timeout.

Observed check-run evidence (run 35252315921, head fdd5122)

This branch contains only .github/workflows/ci.yml; the monorepo scaffold (feat/monorepo-scaffold, todo_8cISFrEW) has not merged yet, so there is no root package.json / pnpm-lock.yaml on master:

Step	Result	Why
Checkout, Node 20 setup	✅ success	—
Enable pnpm via corepack	✅ success	Workflow mechanism verified live
pnpm store path + cache restore	✅ success	—
pnpm install --frozen-lockfile	❌ failure — expected	ERR_PNPM_NO_LOCKFILE: no scaffold yet, so no root package.json/pnpm-lock.yaml
pnpm turbo run typecheck / test / build	⏸ skipped	Depend on install; turbo tasks come from the scaffold's turbo.json

Plan per the task: the PR stays open, gets rebased onto master once the scaffold merges, and all steps re-run on the fresh SHA.

Note for the scaffold PR: pnpm ≥10 blocks dependency postinstall scripts by default — declare any needed pnpm.onlyBuiltDependencies (e.g. esbuild/sharp) in the root package.json so CI installs cleanly.

How to Review

• Only .github/workflows/ci.yml changed — single job, linear steps, no matrix.
• pnpm turbo run test deliberately has no --filter flags: the scaffold owns package naming, and turbo runs the test task wherever it is defined (domain/extraction).
• Intentionally excluded: a push-to-master trigger (brief scopes to pull_request → master), turbo remote caching (no token available), and job splitting (single-job is fine at this size).

Test Evidence

• Workflow YAML validated locally (parse + structural assertions on triggers, permissions, and all seven steps).
• Live check-run evidence: run 35252315921 — all pnpm/corepack/cache/setup steps green; install fails solely on the missing scaffold manifests (ERR_PNPM_NO_LOCKFILE), typecheck/test/build pending scaffold.

Human author: Gilbert Polanco (gilbertpolanco42@gmail.com)

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