fix: [#285] correct RAG optional-dependency install instructions (cofone -> fenn[rag-*])

[pcbeingused333]

What

The RAG modules told users to pip install "cofone[pdf]" (and [web], [faiss]) in ImportError messages and docstrings. Corrected to the real distribution and extras: fenn[rag-pdf], fenn[rag-web], fenn[rag-faiss].

Why

cofone is not a valid distribution and the extras are named rag-* in pyproject.toml, so the previous commands fail with No matching distribution found.

Scope

10 strings across fenn/agents/rag/loader.py, retriever.py, rag.py. Left the [cofone] log prefixes untouched (separate concern — happy to follow up).

Testing

ruff check and ruff format --check pass; no tests referenced these strings.

Closes #285

[ApusBerliozi]

Looks good for me, but I'm not so sure regarding dependencies...

@blkdmr, could you please also take a look at this one?

[pcbeingused333]

Thanks for the review, @ApusBerliozi! 🙌 This might help: this PR only changes the text in the error messages and docstrings — it doesn't touch pyproject.toml or any actual dependency. The extras rag-pdf, rag-web, rag-faiss already exist in pyproject.toml; the messages were just pointing to names that don't resolve (cofone[pdf]). So after this change, pip install "fenn[rag-pdf]" (and [rag-web] / [rag-faiss]) match the real extras and actually work. Happy to adjust the wording if you'd prefer a different convention.