docs: clarify chromadb retrieval example scope

[rlippmann]

Summary

Clarify the scope of the ChromaDB retrieval-filtering example.

The generic HR retrieval examples now demonstrate both:

• policy-driven eligibility
• premise-driven relevance

The ChromaDB example intentionally demonstrates only policy-driven retrieval filtering through Chroma metadata filters.

This PR updates the documentation so it no longer implies that the ChromaDB example implements the same full retrieval behavior as the generic examples.

Why

The retrieval-filtering examples recently gained premise-driven relevance behavior.

The ChromaDB example was intentionally left unchanged, but the documentation still described it as demonstrating the same behavior as the generic HR retrieval examples.

This PR narrows the documented scope so the examples accurately reflect their current behavior.

Example metadata

• Enforcement point: retrieval filtering
• Domain: HR policy lookup
• Technology/runtime: ChromaDB
• Example type: technology-specific example
• Does this use directive drafter? should normally be no: no
• Does this derive state from model output? must be no for primary examples: no

Checklist

• Repository scaffolding / governance change
• One primary enforcement point
• Explicit authoritative state
• No model-derived state mutation
• Adversarial stub or equivalent test
• Observable runtime behavior change
• Docs updated
• Tests/smoke checks added where practical

Notes

Documentation only.

Updated files:

• python/examples/retrieval_filtering/README.md
• python/examples/retrieval_filtering/chromadb_hr_policy_lookup/README.md

Validation performed:

• npx --yes markdownlint-cli2 python/examples/retrieval_filtering/README.md python/examples/retrieval_filtering/chromadb_hr_policy_lookup/README.md

Result:

• Passed: 0 error(s)

No code, tests, fixtures, or runtime behavior were changed.