rag-followups.md

@mantle/engine	minor

RAG follow-ups on #153:

• kb/query metadata filtering — an optional filter object restricts the search to rows whose metadata contains it, via the JSONB containment operator (metadata @> $2::jsonb). The whole filter binds as a single parameter (no injection surface); a custom metadata_column is supported. Scope retrieval to a source, team, or tag alongside the vector search.
• Separator-aware recursive chunking — text/chunk gains unit: recursive, which walks a separator hierarchy (paragraph → line → sentence → word → character) so chunks break on natural boundaries instead of mid-sentence, then merges pieces up to chunk_size with chunk_overlap characters carried between chunks. The rag-ingest example now uses it.
• Cohere Bedrock embeddings — ai/embed (provider bedrock) now supports cohere.embed-english-v3 and cohere.embed-multilingual-v3 in addition to Amazon Titan. Cohere batches natively (up to 96 texts per call) and takes an input_type (search_document default / search_query / classification / clustering).

Token-accurate chunking remains a follow-up on #153.