Allow overriding ldk-node fork/branch from PR description

Parse the PR body for a `ldk-node: owner/repo#branch` line and use it
to check out a custom fork/branch for the integration test. Falls back
to `lightningdevkit/ldk-node` at its default branch when not specified
or on push events.

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

Author: joostjager

.github/workflows/ldk-node-integration.yml

@@ -15,10 +15,22 @@ jobs:
         uses: actions/checkout@v3
         with:
           path: rust-lightning
+      - name: Determine LDK Node repo and branch
+        id: ldk-node-ref
+        if: github.event_name == 'pull_request'
+        env:
+          PR_BODY: ${{ github.event.pull_request.body }}
+        run: |
+          LDK_NODE_REF=$(echo "$PR_BODY" | sed -n 's/^[[:space:]]*ldk-node:[[:space:]]*//p' | tr -d '[:space:]')
+          if [ -n "$LDK_NODE_REF" ]; then
+            echo "repo=$(echo "$LDK_NODE_REF" | cut -d'#' -f1)" >> "$GITHUB_OUTPUT"
+            echo "ref=$(echo "$LDK_NODE_REF" | cut -d'#' -s -f2)" >> "$GITHUB_OUTPUT"
+          fi
       - name: Checkout LDK Node
         uses: actions/checkout@v3
         with:
-          repository: lightningdevkit/ldk-node
+          repository: ${{ steps.ldk-node-ref.outputs.repo || 'lightningdevkit/ldk-node' }}
+          ref: ${{ steps.ldk-node-ref.outputs.ref || '' }}
           path: ldk-node
       - name: Install Rust stable toolchain
         run: |