fix: materialize internal import endpoints in monorepos

[hsneves]

Summary

• resolve Python imports by physical module path within workspace-aware monorepos
• support namespace packages and .pyi modules without creating external stub nodes
• classify dangling endpoints as external, unresolved internal, malformed, or unclassified
• keep legacy dangling_endpoint_edges while excluding expected external imports from health warnings
• mark unresolved package-manifest and JS/TS package imports as external

Validation

• ruff check graphify tests/test_endpoint_classification.py tests/test_src_layout_import_resolution.py tests/test_multigraph_diagnostics.py
• python -m tools.skillgen --check
• pytest -q: 3588 passed, 33 skipped
• 698-file mixed monorepo: unresolved internal 73 -> 0, malformed 0, unclassified 0; 2495 expected external endpoints classified