D20 — Embedding: interface + Ollama adapter, wired

(Superseded by D135: the embedder and semantic search were removed.) internal/embed: Embedder interface with Embed(text) → Vector. OllamaEmbedder adapter (HTTP POST /api/embed). In-memory Store with cosine similarity. Activated with --ollama <url> (or CARTOGRAPHER_OLLAMA): the search tool supports use_semantic=true for hybrid mode (keyword + vector); index_rebuild also rebuilds the vector store.