fix: support custom config directories via RNS.Reticulum.configdir

The BLE interface now dynamically resolves the interface directory
by checking RNS.Reticulum.configdir when loaded via exec() by Reticulum.
This allows users to specify custom config directories using the
--config flag without encountering import errors.

Changes:
- Update BLEInterface.py to use RNS.Reticulum.configdir when available
- Add fallback to default ~/.reticulum/interfaces for backward compatibility
- Add comprehensive test coverage for config directory resolution
- Update documentation to mention custom config directory support

Fixes #2

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

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

examples/config_example.toml

@@ -1,5 +1,8 @@
 # Example Reticulum configuration with BLE interface
 # Place this in ~/.reticulum/config or /etc/reticulum/config
+#
+# Note: If you use a custom config directory with `rnsd --config /custom/path`,
+# the BLE interface will automatically detect and use that location.
 
 [reticulum]
 enable_transport = No