feat: add `find_context_attributions()` intrinsic function

[dennislwei]

Add find_context_attributions() to mellea/stdlib/components/intrinsic/core.py as a high-level API for the context-attribution intrinsic. The function finds context sentences (in prior conversation messages and RAG documents) that were most important to the LLM in generating each response sentence.

Changes:

• mellea/stdlib/components/intrinsic/core.py: add find_context_attributions()
• mellea/backends/adapters/catalog.py: add context-attribution catalog entry pointing to ibm-granite/granitelib-core-r1.0
• docs/examples/intrinsics/context_attribution.py: usage example
• test/stdlib/components/intrinsic/test_core.py: add test_find_context_attributions with test data