fix: resolve pixi dependency resolution hanging issue

## Dependency Resolution Fix:
- Removed diff-cover from pyproject.toml dependencies (causing hanging)
- Modified quality-metrics workflow to install diff-cover separately via pip
- Split diff-cover installation into separate step to avoid conflicts

## Verification:
- ✅ pixi install -e ci now completes successfully
- ✅ ci-install task works correctly
- ✅ All CI environments can be installed without hanging

This resolves the "Setup pixi" failures that were causing exit code timeouts
in CI workflows. diff-cover is still available but installed at runtime.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

Author: MementoRC

.github/workflows/quality-metrics.yml

@@ -68,10 +68,16 @@ jobs:
             .coverage*
             pytest-report.json
             pytest-report.html
-      - name: diff-cover (PR only)
+      - name: Install diff-cover separately (PR only)
         if: github.event_name == 'pull_request'
         run: |
-          # Use pixi to run diff-cover
+          # Install diff-cover separately to avoid dependency resolution issues
+          pixi run -e ci pip install diff-cover
+
+      - name: diff-cover analysis (PR only)
+        if: github.event_name == 'pull_request'
+        run: |
+          # Use pixi environment to run diff-cover
           git fetch origin main:refs/remotes/origin/main
           pixi run -e ci diff-cover coverage.xml --compare-branch=origin/main --fail-under=40 --html-report diffcover.html --markdown-report diffcover.md --json-report diffcover.json
       - name: Upload diff-cover artifacts

pyproject.toml

@@ -116,7 +116,7 @@ pip-audit = ">=2.6.0"
 
 # Coverage Tools
 coverage = ">=7.0.0"
-diff-cover = ">=7.0.0"
+# diff-cover = "*"  # Install separately in CI workflow due to dependency resolution issues
 
 # ===== LEGACY DEVELOPMENT FEATURES =====
 # Keep existing dev feature for specialized tools