Refactor CI harness script to standardize `TMP_DIR` and token handling

# Summary

Currently, our harness test script initializes `data_dir` directories and tokens inconsistently, making CI tests harder to maintain.

We should refactor `harness.sh` to:

- [ ] Always use `$TMP_DIR` for `data_dir`, logs, and tokens
- [ ] Standardize environment variable initialization across all functions (`forest_init*`, `forest_run_node_*`)
- [ ] Reduce redundant path handling and improve readability

# Completion Criteria

- All `forest_init_*` functions use `$TMP_DIR` for data, logs, and token
- Admin token is exported consistently
- Tests run without modifying paths or tokens

# Additional Links & Resources