ci: make agent PR exploration trusted checkout lightweight (#3071)

The "Checkout trusted base scripts" step did a full actions/checkout of
this large repo on the self-hosted runner. On a recent run it stalled in
the initial `git fetch --depth=1 origin <sha>` for many minutes before
the agent script ever started, and the run had to be cancelled.

The trusted host side only needs the self-contained
`.github/scripts/agent-pr-explore-sandbox.sh`; PR code is checked out
inside Docker and PR context is gathered via the API. Replace the full
checkout with a single-file fetch via `gh api` (raw), pinned to the same
trusted base/dispatch commit, which avoids the git-protocol fetch of the
whole repo entirely.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.github/workflows/agent-pr-explore-sandbox.yml

@@ -28,11 +28,25 @@ jobs:
     timeout-minutes: 45
 
     steps:
-      - name: Checkout trusted base scripts
+      - name: Fetch trusted base script
-        uses: actions/checkout@v6.0.2
+        # Only the self-contained sandbox script is needed on the trusted host;
-        with:
+        # PR code is checked out inside Docker. A full actions/checkout of this
-          ref: ${{ github.event.pull_request.base.sha || github.sha }}
+        # large repo stalled on the self-hosted runner before the agent ever
-          persist-credentials: false
+        # ran, so fetch just the one trusted file via the API instead. The ref
+        # is pinned to the trusted base/dispatch commit, never PR head.
+        shell: bash
+        env:
+          GH_TOKEN: ${{ github.token }}
+          TRUSTED_REF: ${{ github.event.pull_request.base.sha || github.sha }}
+        run: |
+          set -euo pipefail
+          mkdir -p .github/scripts
+          gh api \
+            -H 'Accept: application/vnd.github.raw' \
+            "repos/$GITHUB_REPOSITORY/contents/.github/scripts/agent-pr-explore-sandbox.sh?ref=$TRUSTED_REF" \
+            > .github/scripts/agent-pr-explore-sandbox.sh
+          chmod +x .github/scripts/agent-pr-explore-sandbox.sh
+          echo "Fetched trusted agent-pr-explore-sandbox.sh at $TRUSTED_REF"
 
       - name: Resolve PR metadata
         id: pr