ci: cache the pnpm store and Turborepo output

The workflow had no caching at all, so every job downloaded the whole
dependency graph from scratch and every job rebuilt every package from
scratch, even when nothing relevant had changed.

Extract the repeated setup into a composite action that installs pnpm
before Node (so actions/setup-node can locate the pnpm store to cache),
installs with --frozen-lockfile, and restores .turbo/cache. Turborepo
only reuses output when a task's inputs hash the same, so the turbo cache
cannot go stale.

On the last green run this restores the pnpm store in the four heavy jobs
(cold installs took 7s-22s each) and gives repeat runs, including every
push to a PR branch, warm build and test results.

Author: im10furry

.github/actions/setup/action.yml

@@ -0,0 +1,26 @@
+name: Setup
+description: Install Node.js, pnpm and workspace dependencies, with the pnpm store and Turborepo caches restored.
+
+inputs:
+  node-version:
+    description: Node.js version to install
+    required: true
+
+runs:
+  using: composite
+  steps:
+    # install pnpm first so actions/setup-node can locate the store to cache
+    - uses: pnpm/action-setup@v5
+    - uses: actions/setup-node@v6
+      with:
+        node-version: ${{ inputs.node-version }}
+        cache: pnpm
+    - run: pnpm install --frozen-lockfile
+      shell: bash
+    # Turborepo reuses task output when a task's inputs hash the same; caching
+    # .turbo/cache lets later runs (and later commits) skip rebuilds.
+    - uses: actions/cache@v4
+      with:
+        path: .turbo/cache
+        key: turbo-${{ runner.os }}-${{ inputs.node-version }}-${{ github.sha }}
+        restore-keys: turbo-${{ runner.os }}-${{ inputs.node-version }}-

.github/workflows/ci.yml

@@ -16,12 +16,9 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v6
-      - uses: actions/setup-node@v6
+      - uses: ./.github/actions/setup
         with:
           node-version: latest
-      - uses: pnpm/action-setup@v5
-        with:
-          run_install: true
       - run: pnpm run lint
   test-node-versions:
     runs-on: ubuntu-latest
@@ -30,12 +27,9 @@ jobs:
         node-version: [22, 24]
     steps:
       - uses: actions/checkout@v6
-      - uses: actions/setup-node@v6
+      - uses: ./.github/actions/setup
         with:
           node-version: ${{ matrix.node-version }}
-      - uses: pnpm/action-setup@v5
-        with:
-          run_install: true
       - run: pnpm test
   test-e2e:
     runs-on: ubuntu-latest
@@ -54,12 +48,9 @@ jobs:
     timeout-minutes: 15
     steps:
       - uses: actions/checkout@v6
-      - uses: actions/setup-node@v6
+      - uses: ./.github/actions/setup
         with:
           node-version: 22
-      - uses: pnpm/action-setup@v5
-        with:
-          run_install: true
       - run: pnpm run test-e2e
         env:
           # Firefox refuses to start as root unless $HOME is owned by root, and
@@ -70,21 +61,15 @@ jobs:
     runs-on: macos-latest
     steps:
       - uses: actions/checkout@v6
-      - uses: actions/setup-node@v6
+      - uses: ./.github/actions/setup
         with:
           node-version: latest
-      - uses: pnpm/action-setup@v5
-        with:
-          run_install: true
       - run: pnpm test
   test-windows:
     runs-on: windows-latest
     steps:
       - uses: actions/checkout@v6
-      - uses: actions/setup-node@v6
+      - uses: ./.github/actions/setup
         with:
           node-version: latest
-      - uses: pnpm/action-setup@v5
-        with:
-          run_install: true
       - run: pnpm test