docs: add metadata filtering implementation plan

docs: add metadata filtering design spec (Stage 17)

docs: add citation highlighting implementation plan

docs: add citation highlighting design spec

docs: streaming responses implementation plan

docs: streaming responses design spec

docs: async embed upload design spec

docs: contextual retrieval implementation plan

docs: contextual retrieval design spec (eval-only, v1.3.0 target)

docs: add Prism v3 implementation plan (8 tasks: rebrand + URL ingest + briefing + citations + multi-workspace)

docs: add Prism v3 design spec (domain-agnostic rebrand + URL ingestion + briefing + citations + multi-workspace)