fix(workflows): address Copilot review feedback

Fixes 3 issues identified by Copilot code review:

1. **sync-labels.yml (spec detection)**: Use GitHub event context
   - Changed from `git diff HEAD~1 HEAD` to `${{ github.event.before/after }}`
   - Handles edge cases: first commit, force pushes, rebases
   - Added fallback for initial push (SHA = 0000...)

2. **sync-labels.yml (impl detection)**: Same fix applied
   - Uses GitHub event context for reliability
   - Prevents failures on edge cases

3. **impl-generate.yml (critical bug)**: Fixed file verification logic
   - Claude already commits implementation file earlier in workflow
   - Changed verification from "is file staged?" to "does file exist?"
   - Uses `git ls-files --error-unmatch` to verify implementation
   - Only stages and verifies metadata file (which we create)
   - Prevents false "not staged" errors that would block all workflows

Updated CLAUDE.md to reflect the corrected workflow behavior.

Author: MarkusNeusinger

.github/workflows/impl-generate.yml

@@ -526,27 +526,35 @@ jobs:
               yaml.dump(data, f, default_flow_style=False, sort_keys=False, allow_unicode=True)
           "
 
-          # Commit and push (ATOMIC: both implementation AND metadata together)
+          # Commit and push metadata (implementation already committed by Claude)
           git config user.name "github-actions[bot]"
           git config user.email "github-actions[bot]@users.noreply.github.com"
 
           IMPL_FILE="plots/${SPEC_ID}/implementations/${LIBRARY}.py"
 
-          # Add both files in a single commit to prevent partial merges
-          git add "$METADATA_FILE" "$IMPL_FILE"
+          # Verify implementation file exists in the repository (Claude should have committed it)
+          if ! git ls-files --error-unmatch "$IMPL_FILE" >/dev/null 2>&1; then
+            echo "::error::Implementation file not found in repository - cannot commit"
+            echo "::error::Expected implementation at: $IMPL_FILE"
+            echo "::error::This indicates Claude failed to create the implementation file"
+            exit 1
+          fi
+
+          # Add metadata file
+          git add "$METADATA_FILE"
 
-          # Verify both files are staged
-          if ! git diff --cached --name-only | grep -q "${LIBRARY}.py"; then
-            echo "::error::Implementation file not staged - cannot commit"
-            echo "::error::This indicates the implementation file is missing or not modified"
+          # Verify metadata file is staged
+          if ! git diff --cached --name-only | grep -q "$(basename "$METADATA_FILE")"; then
+            echo "::error::Metadata file not staged - cannot commit"
+            echo "::error::This indicates the metadata file was not added correctly"
             exit 1
           fi
 
-          git commit -m "feat(${LIBRARY}): implement ${SPEC_ID} with metadata"
+          git commit -m "chore(${LIBRARY}): add metadata for ${SPEC_ID}"
           git push origin "$BRANCH"
 
           echo "::notice::Created metadata file: $METADATA_FILE (py${PYTHON_VERSION}, ${LIBRARY}==${LIBRARY_VERSION})"
-          echo "::notice::Committed implementation and metadata atomically"
+          echo "::notice::Implementation verified present, metadata committed"
 
       # ========================================================================
       # Process images: optimize + thumbnail

.github/workflows/sync-labels.yml

@@ -29,11 +29,23 @@ jobs:
 
       - name: Detect new specifications merged to main
         id: detect
+        env:
+          BEFORE: ${{ github.event.before }}
+          AFTER: ${{ github.event.after }}
         run: |
           # Get list of specifications added/modified in this push
-          CHANGED_SPECS=$(git diff --name-only HEAD~1 HEAD | \
-            grep -oP 'plots/\K[^/]+(?=/specification\.(md|yaml))' | \
-            sort -u)
+          # Use GitHub event context for reliability (handles first push, force push, etc.)
+          if [ "$BEFORE" = "0000000000000000000000000000000000000000" ]; then
+            # First push or new branch: diff-tree on the after commit only
+            CHANGED_SPECS=$(git diff-tree --no-commit-id --name-only -r "$AFTER" | \
+              grep -oP 'plots/\K[^/]+(?=/specification\.(md|yaml))' | \
+              sort -u || true)
+          else
+            # Normal push: diff between before and after SHAs
+            CHANGED_SPECS=$(git diff --name-only "$BEFORE" "$AFTER" | \
+              grep -oP 'plots/\K[^/]+(?=/specification\.(md|yaml))' | \
+              sort -u || true)
+          fi
 
           if [ -z "$CHANGED_SPECS" ]; then
             echo "::notice::No specification changes detected"
@@ -102,10 +114,21 @@ jobs:
 
       - name: Detect new implementations merged to main
         id: detect
+        env:
+          BEFORE: ${{ github.event.before }}
+          AFTER: ${{ github.event.after }}
         run: |
           # Get list of implementations added/modified in this push
-          CHANGED_IMPLS=$(git diff --name-only HEAD~1 HEAD | \
-            grep -P 'plots/[^/]+/implementations/[^/]+\.py$')
+          # Use GitHub event context for reliability (handles first push, force push, etc.)
+          if [ "$BEFORE" = "0000000000000000000000000000000000000000" ]; then
+            # First push or new branch: diff-tree on the after commit only
+            CHANGED_IMPLS=$(git diff-tree --no-commit-id --name-only -r "$AFTER" | \
+              grep -P 'plots/[^/]+/implementations/[^/]+\.py$' || true)
+          else
+            # Normal push: diff between before and after SHAs
+            CHANGED_IMPLS=$(git diff --name-only "$BEFORE" "$AFTER" | \
+              grep -P 'plots/[^/]+/implementations/[^/]+\.py$' || true)
+          fi
 
           if [ -z "$CHANGED_IMPLS" ]; then
             echo "::notice::No implementation changes detected"

CLAUDE.md

@@ -659,7 +659,7 @@ impl-generate.yml
     ├── Creates branch: implementation/{specification-id}/{library}
     ├── Generates implementation code
     ├── Tests with MPLBACKEND=Agg
-    ├── Commits implementation + metadata atomically (prevents partial PRs)
+    ├── Verifies implementation exists before committing metadata
     ├── Uploads preview to GCS staging
     ├── Creates PR: implementation/{specification-id}/{library} → main
     └── Triggers impl-review.yml