chore: split heavy ML stack into optional mmory extras

[AaineeSinha]

Closes #35

This PR separates the heavy machine learning packages (chromadb, sentence-transformers, torch) out of the main requirements.txt file and moves them into a new requirements-memory.txt file.

This ensures that the core CLI installation remains lightweight and fast for new users and CI environments while preserving an explicit installation path for future vector memory features.

[rosspeili]

Thanks for this as well @AaineeSinha.

Before merge, could you fix a couple small README things in the same PR?

• Close the optional pip install -r requirements-memory.txt code block before ### 2. Configure defaults (looks like the fence is missing right now).
• Tidy the project structure line for requirements-memory.txt (duplicate comment on one line).
• Optional: link to feat: long-term agent memory across sessions (sidecar storage) #13 where you mention long-term / vector memory.

CI should be fine as-is since it only installs requirements.txt. Thanks again!