[CI Deduplication] External repo detection module (ROCm#3181)

Foundational module for external repository integration (ROCm#2960). Provides configuration lookup and path resolution for rocm-libraries and rocm-systems. Outputs GitHub Actions variables (cmake_source_var, submodule_path, fetch_exclusion, optional extra_cmake_options). Supports dynamic loading of therock_configure_ci.py and therock_matrix.py for skip patterns and test lists. Includes unit tests in test_detect_external_repo_config.py. Workflow integration to follow in later PRs.

Author: jayhawk-commits