feat: add BASIC_MEMORY_CONFIG_DIR environment variable support

Add support for overriding config directory location via
BASIC_MEMORY_CONFIG_DIR environment variable. This enables
cloud deployments to store config.json in persistent storage
(e.g., Tigris) while keeping memory.db on local ephemeral disk.

Changes:
- ConfigManager now checks BASIC_MEMORY_CONFIG_DIR before using default
- Add tests verifying custom config directory works correctly
- All existing tests pass

Implements Phase 1 of SPEC-15.

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

Co-Authored-By: Claude <noreply@anthropic.com>
Signed-off-by: phernandez <paul@basicmachines.co>

Author: phernandez

src/basic_memory/config.py

@@ -251,7 +251,12 @@ def __init__(self) -> None:
         if isinstance(home, str):
             home = Path(home)
 
-        self.config_dir = home / DATA_DIR_NAME
+        # Allow override via environment variable
+        if config_dir := os.getenv("BASIC_MEMORY_CONFIG_DIR"):
+            self.config_dir = Path(config_dir)
+        else:
+            self.config_dir = home / DATA_DIR_NAME
+
         self.config_file = self.config_dir / CONFIG_FILE_NAME
 
         # Ensure config directory exists

tests/test_config.py

@@ -179,3 +179,28 @@ def test_disable_permalinks_flag_can_be_enabled(self):
         """Test that disable_permalinks flag can be set to True."""
         config = BasicMemoryConfig(disable_permalinks=True)
         assert config.disable_permalinks is True
+
+    def test_config_manager_respects_custom_config_dir(self, monkeypatch):
+        """Test that ConfigManager respects BASIC_MEMORY_CONFIG_DIR environment variable."""
+        with tempfile.TemporaryDirectory() as temp_dir:
+            custom_config_dir = Path(temp_dir) / "custom" / "config"
+            monkeypatch.setenv("BASIC_MEMORY_CONFIG_DIR", str(custom_config_dir))
+
+            config_manager = ConfigManager()
+
+            # Verify config_dir is set to the custom path
+            assert config_manager.config_dir == custom_config_dir
+            # Verify config_file is in the custom directory
+            assert config_manager.config_file == custom_config_dir / "config.json"
+            # Verify the directory was created
+            assert config_manager.config_dir.exists()
+
+    def test_config_manager_default_without_custom_config_dir(self, config_home, monkeypatch):
+        """Test that ConfigManager uses default location when BASIC_MEMORY_CONFIG_DIR is not set."""
+        monkeypatch.delenv("BASIC_MEMORY_CONFIG_DIR", raising=False)
+
+        config_manager = ConfigManager()
+
+        # Should use default location
+        assert config_manager.config_dir == config_home / ".basic-memory"
+        assert config_manager.config_file == config_home / ".basic-memory" / "config.json"