Promote __len__ and add clear() to the Retriever ABC

[adaumsilva]

Description

All three built-in retrievers implement __len__, but it is not part of the Retriever ABC, so generic code (if len(pipeline.retriever) == 0: ...) is not type-safe and third-party retrievers are not required to provide it. There is also no way to empty an index short of constructing a new retriever.

Acceptance criteria

• Retriever.__len__(self) -> int declared @abstractmethod in ragframework/base.py with a docstring ("number of indexed chunks")
• Retriever.clear(self) -> None declared @abstractmethod; implemented in InMemoryRetriever (reset list + matrix), FAISSRetriever (drop index + dimension so the next add rebuilds), ChromaRetriever (delete + recreate the collection)
• CONTRIBUTING.md "Adding a New Integration" section updated to list the new required methods
• Tests: clear() → len() == 0 and retrieve() returns []; re-add after clear works (FAISS dimension may differ after clear)
• CHANGELOG.md updated under [Unreleased] → ### Changed (note: breaking for third-party Retriever subclasses)

Files to touch

• ragframework/base.py
• ragframework/retriever/in_memory.py, faiss.py, chroma.py
• CONTRIBUTING.md
• tests/test_retriever/*

Estimated effort: Small (< 2 hours)