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

docs/CLAUDE.md

@@ -326,6 +326,40 @@ print(soup.select_one('main'))
 print(soup.select_one('div[role="main"]'))
 ```
 
+## Running Tests
+
+**IMPORTANT: You must install the package before running tests**
+
+```bash
+# 1. Install package in editable mode (one-time setup)
+pip install -e .
+
+# 2. Run all tests
+pytest
+
+# 3. Run specific test files
+pytest tests/test_config_validation.py
+pytest tests/test_github_scraper.py
+
+# 4. Run with verbose output
+pytest -v
+
+# 5. Run with coverage report
+pytest --cov=src/skill_seekers --cov-report=html
+```
+
+**Why install first?**
+- Tests import from `skill_seekers.cli` which requires the package to be installed
+- Modern Python packaging best practice (PEP 517/518)
+- CI/CD automatically installs with `pip install -e .`
+- conftest.py will show helpful error if package not installed
+
+**Test Coverage:**
+- 391+ tests passing
+- 39% code coverage
+- All core features tested
+- CI/CD tests on Ubuntu + macOS with Python 3.10-3.12
+
 ## Troubleshooting
 
 **No content extracted**: Check `main_content` selector. Common values: `article`, `main`, `div[role="main"]`, `div.content`