feat: Add support for external knowledge directory via CREWAI_KNOWLEDGE_FILE_DIR

- Add get_knowledge_directory() utility function following CREWAI_STORAGE_DIR pattern
- Update BaseFileKnowledgeSource, CrewDoclingSource, and ExcelKnowledgeSource to use new function
- Add comprehensive tests for utility function and knowledge source integration
- Maintain backward compatibility with default 'knowledge' directory
- Add proper error handling for non-existent directories

Fixes #3519

Co-Authored-By: João <joao@crewai.com>

tests/knowledge/test_external_knowledge_directory.py

@@ -0,0 +1,99 @@
+import os
+import tempfile
+from pathlib import Path
+import pytest
+from crewai.knowledge.source.text_file_knowledge_source import TextFileKnowledgeSource
+from crewai.knowledge.source.json_knowledge_source import JSONKnowledgeSource
+
+
+class TestExternalKnowledgeDirectory:
+    def test_text_file_source_with_external_directory(self):
+        """Test that TextFileKnowledgeSource works with external directory."""
+        with tempfile.TemporaryDirectory() as temp_dir:
+            test_file = Path(temp_dir) / "test.txt"
+            test_content = "This is a test file for external knowledge directory."
+            test_file.write_text(test_content)
+            
+            os.environ["CREWAI_KNOWLEDGE_FILE_DIR"] = temp_dir
+            try:
+                source = TextFileKnowledgeSource(file_paths=["test.txt"])
+                
+                assert len(source.content) == 1
+                loaded_content = list(source.content.values())[0]
+                assert loaded_content == test_content
+                
+            finally:
+                del os.environ["CREWAI_KNOWLEDGE_FILE_DIR"]
+
+    def test_json_file_source_with_external_directory(self):
+        """Test that JSONKnowledgeSource works with external directory."""
+        with tempfile.TemporaryDirectory() as temp_dir:
+            test_file = Path(temp_dir) / "test.json"
+            test_data = {"name": "John", "age": 30, "city": "New York"}
+            import json
+            test_file.write_text(json.dumps(test_data))
+            
+            os.environ["CREWAI_KNOWLEDGE_FILE_DIR"] = temp_dir
+            try:
+                source = JSONKnowledgeSource(file_paths=["test.json"])
+                
+                assert len(source.content) == 1
+                loaded_content = list(source.content.values())[0]
+                assert "John" in loaded_content
+                assert "30" in loaded_content
+                assert "New York" in loaded_content
+                
+            finally:
+                del os.environ["CREWAI_KNOWLEDGE_FILE_DIR"]
+
+    def test_knowledge_source_fallback_to_default(self):
+        """Test that knowledge sources fall back to default directory when env var not set."""
+        if "CREWAI_KNOWLEDGE_FILE_DIR" in os.environ:
+            del os.environ["CREWAI_KNOWLEDGE_FILE_DIR"]
+        
+        knowledge_dir = Path("knowledge")
+        knowledge_dir.mkdir(exist_ok=True)
+        test_file = knowledge_dir / "test_fallback.txt"
+        test_content = "This is a test file for default knowledge directory."
+        
+        try:
+            test_file.write_text(test_content)
+            
+            source = TextFileKnowledgeSource(file_paths=["test_fallback.txt"])
+            
+            assert len(source.content) == 1
+            loaded_content = list(source.content.values())[0]
+            assert loaded_content == test_content
+            
+        finally:
+            if test_file.exists():
+                test_file.unlink()
+
+    def test_knowledge_source_with_absolute_path_ignores_env_var(self):
+        """Test that absolute paths ignore the environment variable."""
+        with tempfile.TemporaryDirectory() as temp_dir:
+            test_file = Path(temp_dir) / "test_absolute.txt"
+            test_content = "This is a test file with absolute path."
+            test_file.write_text(test_content)
+            
+            with tempfile.TemporaryDirectory() as other_dir:
+                os.environ["CREWAI_KNOWLEDGE_FILE_DIR"] = other_dir
+                try:
+                    source = TextFileKnowledgeSource(file_paths=[str(test_file)])
+                    
+                    assert len(source.content) == 1
+                    loaded_content = list(source.content.values())[0]
+                    assert loaded_content == test_content
+                    
+                finally:
+                    del os.environ["CREWAI_KNOWLEDGE_FILE_DIR"]
+
+    def test_knowledge_source_error_with_invalid_external_directory(self):
+        """Test that proper error is raised when external directory doesn't exist."""
+        invalid_dir = "/path/that/does/not/exist"
+        os.environ["CREWAI_KNOWLEDGE_FILE_DIR"] = invalid_dir
+        try:
+            with pytest.raises(ValueError, match="Knowledge directory does not exist"):
+                TextFileKnowledgeSource(file_paths=["test.txt"])
+        finally:
+            del os.environ["CREWAI_KNOWLEDGE_FILE_DIR"]