feat: general_search recipe layer — relevance-ranked session-agnostic context #65

Add a new general_search layer kind to the recipe system. Runs hybrid vector+FTS5 search across summaries and documents tables using the current session's latest user turn (plus 1-2 recent turn summaries) as the query. Returns the top N results that were not already emitted by earlier session-scoped layers. This is session-agnostic — results may come from any session, including older turns of the current one. Intent: surface unexpected but semantically relevant context to the model ("insight by association"). Layer JSON: {"kind": "general_search", "limit": 3}