Fix CI: install from repo checkout instead of PyPI

Package isn't published to PyPI yet, so CI needs to install
from the checked-out source with pip install -e .
Also adds test step to CI pipeline.

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

Author: Supratim Dam

.github/workflows/growthlint.yml

@@ -18,7 +18,10 @@ jobs:
           python-version: "3.12"
 
       - name: Install GrowthLint
-        run: pip install growthlint
+        run: pip install -e .
+
+      - name: Run tests
+        run: python -m pytest tests/ -q
 
       - name: Run GrowthLint scan
         run: growthlint scan . --format json --output growthlint-report.json

README.md

@@ -269,7 +269,7 @@ jobs:
       - uses: actions/setup-python@v5
         with:
           python-version: "3.12"
-      - run: pip install growthlint
+      - run: pip install -e .
       - run: growthlint check-pr . --min-score 60
 ```