[FEATURE]: Full portable mode. #4526

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Currently it is difficult to make opencode portable and use same config and sessions on multiple devices.

Env var OPENCODE_CONFIG_DIR allows relocating config directory.

But data directory located at ~/.local/share/opencode cannot be relocated.

Proposal: Add OPENCODE_DATA_DIR env var to override data directory.

This will allow us to go full portable mode by exporting both config and data directory to a single location.