fix(core): consolidate DOC_EXTENSIONS copies and add drift guard (#1489)

[rishu685]

Closes #1489

Context

Two hand-written literal copies of the documentation extension set ({".md", ".mdx", ".rst", ".txt", ".adoc"}) remained in categories.py and fix_shape.py, with no architectural test in place to prevent future drift.

Solution

1. Consolidated Copies:
   • Pointed _DOC_SUFFIXES in packages/core/src/repowise/core/generation/categories.py to support_paths.DOC_EXTENSIONS.
   • Pointed _DOC_EXT in packages/core/src/repowise/core/ingestion/git_indexer/fix_shape.py to support_paths.DOC_EXTENSIONS.
2. Added Drift Guard Test:
   • Created tests/unit/test_no_doc_extension_copies.py (following test_no_test_path_copies.py) to inspect the AST across packages/ and prevent new hand-written doc-extension sets or constants from being introduced outside repowise.core.support_paths.
   • Verified that categories._DOC_SUFFIXES, fix_shape._DOC_EXT, and knowledge_graph._DOC_EXTENSIONS all alias the exact same DOC_EXTENSIONS object.

Verification

pytest tests/unit/test_no_doc_extension_copies.py tests/unit/test_support_paths.py tests/unit/generation/test_categories.py tests/unit/ingestion/test_fix_shape.py
# 58 passed in 6.02s

ruff check packages/core/src/repowise/core/generation/categories.py packages/core/src/repowise/core/ingestion/git_indexer/fix_shape.py tests/unit/test_no_doc_extension_copies.py
# All checks passed!