D135 — Remove semantic search and the Ollama embedding backend

Decision. Semantic and hybrid search are removed, along with internal/embed, the embeddings half of the persisted index, the search.ollama_* configuration, the --ollama flag and the CARTOGRAPHER_OLLAMA* environment variables. search accepts query, scope and limit; mode and use_semantic are gone from the input schema and a call passing either is rejected with an error naming the removal, rather than silently downgraded to keyword results. Three invariants hold: keyword behaviour is unchanged (the FTS5 trigram path, the all-terms-then-any-term retry, the title/snippet enrichment of D70, and the reported keyword_fts5/keyword mode values); search keeps its query and scope semantics; and a database that still carries an embeddings table opens and works untouched — the table is simply never read or written again, and CREATE TABLE is dropped from schema init so new databases never get one.

Rationale. The capability was opt-in from the start and the only deployment that ever enabled it removed the backend the same day: a full rebuild against a GPU-backed Ollama OOM-killed the container at both 2 and 3 GiB and persisted 0 of 142 embeddings. It was not free while disabled either — a whole package, a second input schema, a duplicated handler, four accessor methods on the persisted index, four configuration knobs, and a search contract advertising two modes that error out at call time on every deployment. Removing it rather than deprecating it is the right call at 0.x: there is no supported deployment to stay compatible with, and rejecting the removed arguments makes a stale client fail loudly instead of quietly getting results it did not ask for.