fix(ci): consolidate black version — drop workflow pin, floor to >=24.0.0

The CI workflow hard-pinned black==24.10.0 separately from requirements.txt
(which said >=23.0.0), causing version skew that reformatted 11 QMD files
on every CI run. Remove the override and let requirements.txt be the single
source of truth, bumped to >=24.0.0 to align with current latest.

.github/workflows/book-validate-dev.yml

@@ -102,9 +102,6 @@ jobs:
           echo "🔍 Installing and running pre-commit..."
           python -m pip install --upgrade pip
           pip install pre-commit
-          # Keep formatter output deterministic across local/CI runs.
-          pip install black==24.10.0
-          
           echo "📦 Installing project dependencies for custom hooks..."
           pip install -r requirements.txt || echo "⚠️ Could not install all requirements, continuing..."
           echo "📦 Installing fallback hook dependencies..."

book/tools/dependencies/requirements.txt

@@ -28,7 +28,7 @@ ollama>=0.3.0
 betterbib>=7.5.0
 # Reference-check helpers (optional tooling)
 bibtexparser>=1.4.0
-black>=23.0.0
+black>=24.0.0
 # PyGithub>=1.55 # Rarely used - only for contributor update workflow
 
 # --- Heavy Development & ML Dependencies ---