Revert CI workflow changes from #1224

Roll back actions/checkout v6.0.2 to v4, restore oven-sh/setup-bun
action, and remove explicit ref parameter from compat workflow.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.github/workflows/ci-macos-compat.yml

@@ -17,9 +17,8 @@ jobs:
     runs-on: ${{ matrix.os }}
     steps:
       - name: Checkout
-        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+        uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4
         with:
-          ref: ${{ github.event_name == 'pull_request' && github.event.pull_request.head.sha || github.sha }}
           submodules: recursive
 
       - name: Select Xcode

.github/workflows/ci.yml

@@ -11,7 +11,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
-        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+        uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4
 
       - name: Validate Depot runner guards
         run: ./tests/test_ci_self_hosted_guard.sh
@@ -35,13 +35,10 @@ jobs:
         working-directory: web
     steps:
       - name: Checkout
-        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+        uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4
 
-      - name: Install Bun
-        run: |
-          set -euo pipefail
-          curl -fsSL https://bun.sh/install | bash -s -- bun-v1.3.10
-          echo "$HOME/.bun/bin" >> "$GITHUB_PATH"
+      - name: Setup Bun
+        uses: oven-sh/setup-bun@3d267786b128fe76c2f16a390aa2448b815359f3 # v2
 
       - name: Install dependencies
         run: bun install --frozen-lockfile
@@ -53,7 +50,7 @@ jobs:
     runs-on: macos-15
     steps:
       - name: Checkout
-        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+        uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4
         with:
           submodules: recursive
 
@@ -160,7 +157,7 @@ jobs:
     runs-on: depot-macos-latest
     steps:
       - name: Checkout
-        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+        uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4
         with:
           submodules: recursive