fix: Add package installation check and fix test imports (Task 2.1)

Fixes test import errors in 7 test files that failed without package installed.

**Changes:**

1. **tests/conftest.py** - Added pytest_configure() hook
   - Checks if skill_seekers package is installed before running tests
   - Shows helpful error message guiding users to run `pip install -e .`
   - Prevents confusing ModuleNotFoundError during test runs

2. **tests/test_constants.py** - Fixed dynamic imports
   - Changed `from cli import` to `from skill_seekers.cli import` (6 locations)
   - Fixes imports in test methods that dynamically import modules
   - All 16 tests now pass ✅

3. **tests/test_llms_txt_detector.py** - Fixed patch decorators
   - Changed `patch('cli.llms_txt_detector.` to `patch('skill_seekers.cli.llms_txt_detector.` (4 locations)
   - All 4 tests now pass ✅

4. **docs/CLAUDE.md** - Added "Running Tests" section
   - Clear instructions on installing package before testing
   - Explanation of why installation is required
   - Common pytest commands and options
   - Test coverage statistics

**Testing:**
- ✅ All 101 tests pass across the 7 affected files:
  - test_async_scraping.py (11 tests)
  - test_config_validation.py (26 tests)
  - test_constants.py (16 tests)
  - test_estimate_pages.py (8 tests)
  - test_integration.py (23 tests)
  - test_llms_txt_detector.py (4 tests)
  - test_llms_txt_downloader.py (13 tests)
- ✅ conftest.py check works correctly
- ✅ Helpful error shown when package not installed

**Impact:**
- Developers now get clear guidance when tests fail due to missing installation
- All test import issues resolved
- Better developer experience for contributors

tests/conftest.py

@@ -2,11 +2,28 @@
 Pytest configuration for tests.
 
 Configures anyio to only use asyncio backend (not trio).
+Checks that the skill_seekers package is installed before running tests.
 """
 
+import sys
 import pytest
 
 
+def pytest_configure(config):
+    """Check if package is installed before running tests."""
+    try:
+        import skill_seekers
+    except ModuleNotFoundError:
+        print("\n" + "=" * 70)
+        print("ERROR: skill_seekers package not installed")
+        print("=" * 70)
+        print("\nPlease install the package in editable mode first:")
+        print("  pip install -e .")
+        print("\nOr activate your virtual environment if you already installed it.")
+        print("=" * 70 + "\n")
+        sys.exit(1)
+
+
 @pytest.fixture(scope="session")
 def anyio_backend():
     """Override anyio backend to only use asyncio (not trio)."""