chore(ci): move checkout out of composite setup action

The composite action at .github/actions/setup previously included
actions/checkout@v6 as its first step, which is structurally impossible:
GitHub Actions must load .github/actions/setup/action.yml from disk
before it can execute the composite, but that file only exists on disk
after actions/checkout has run. Result: every job using the composite
failed with "Can't find 'action.yml' ... Did you forget to run
actions/checkout before running your local action?"

Fix: remove the checkout step from the composite action, and add
- uses: actions/checkout@v6 as the explicit first step of each of the
7 jobs that use the composite (prepare, lint, typecheck, test,
repo-integrity, build, pkg-pr-new).

The composite still centralises pnpm + node + install. The checkout
duplication is unavoidable because of how GitHub Actions resolves
local composite action paths.

Bug introduced in b819734 (Stage 1c). Caught on first post-push
CI run.

Author: johnleider

.github/actions/setup/action.yml

@@ -1,9 +1,8 @@
 name: Setup
-description: Checkout, pnpm, node, and install dependencies
+description: pnpm, node, and install dependencies (assumes repo is already checked out)
 runs:
   using: composite
   steps:
-    - uses: actions/checkout@v6
     - uses: pnpm/action-setup@v4
     - uses: actions/setup-node@v4
       with:

.github/workflows/pr-checks.yml

@@ -10,12 +10,14 @@ jobs:
   prepare:
     runs-on: ubuntu-latest
     steps:
+      - uses: actions/checkout@v6
       - uses: ./.github/actions/setup
 
   lint:
     needs: prepare
     runs-on: ubuntu-latest
     steps:
+      - uses: actions/checkout@v6
       - uses: ./.github/actions/setup
       - name: Run linter
         run: pnpm lint
@@ -24,6 +26,7 @@ jobs:
     needs: prepare
     runs-on: ubuntu-latest
     steps:
+      - uses: actions/checkout@v6
       - uses: ./.github/actions/setup
       - name: Run type checking
         run: pnpm typecheck
@@ -32,6 +35,7 @@ jobs:
     needs: prepare
     runs-on: ubuntu-latest
     steps:
+      - uses: actions/checkout@v6
       - uses: ./.github/actions/setup
       - name: Run tests with coverage
         run: pnpm test:run --coverage
@@ -46,6 +50,7 @@ jobs:
     needs: prepare
     runs-on: ubuntu-latest
     steps:
+      - uses: actions/checkout@v6
       - uses: ./.github/actions/setup
       - name: Run knip check
         run: pnpm repo:knip
@@ -56,6 +61,7 @@ jobs:
     runs-on: ubuntu-latest
     needs: [prepare]
     steps:
+      - uses: actions/checkout@v6
       - uses: ./.github/actions/setup
       - name: Build packages
         run: pnpm build
@@ -115,6 +121,7 @@ jobs:
     needs: [build]
     runs-on: ubuntu-latest
     steps:
+      - uses: actions/checkout@v6
       - uses: ./.github/actions/setup
       - name: Download dist artifact
         uses: actions/download-artifact@v4