ci: update pdm-multirepo to v4

v4 fixes branch name resolution by using commit hashes instead of
branch names in override URLs, which resolves UnpackError failures
during pdm lock.

Co-developed-by: Claude Code v1.0.21 (claude-sonnet-4-20250514)

Author: robtaylor

.github/workflows/coverage.yml

@@ -24,7 +24,7 @@ jobs:
           cache: true
 
       - name: Install dependencies with multirepo
-        uses: chipflow/pdm-multirepo@v3
+        uses: chipflow/pdm-multirepo@v4
 
       - name: Build coverage file
         run: |

.github/workflows/main.yaml

@@ -30,7 +30,7 @@ jobs:
           cache: true
 
       - name: Install dependencies with multirepo
-        uses: chipflow/pdm-multirepo@v3
+        uses: chipflow/pdm-multirepo@v4
 
       - name: Run tests
         run: pdm run test

.github/workflows/preview-docs.yml

@@ -20,7 +20,7 @@ jobs:
           cache: true
 
       - name: Install dependencies with multirepo
-        uses: chipflow/pdm-multirepo@v3
+        uses: chipflow/pdm-multirepo@v4
 
       - name: Build docs
         run: pdm docs

.github/workflows/test-examples.yml

@@ -85,7 +85,7 @@ jobs:
           cache-dependency-path: './**/pyproject.toml'
 
       - name: Install dependencies with multirepo
-        uses: chipflow/pdm-multirepo@v3
+        uses: chipflow/pdm-multirepo@v4
         with:
           working-directory: ${{ env.test_repo_path }}