refactor: data_repository uses _TRCC_PKG from paths.py, drop dupes

_THIS_DIR was _TRCC_PKG by another name, SRC_DIR was dead (zero
callers), PROJECT_ROOT used once. Import _TRCC_PKG from the canonical
source and derive project root from it.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>