feat(engine)!: provider-independent embedding client (RFC-012 Phase 2)

Replace the Gemini-only EmbeddingClient with one resolved EmbeddingConfig { provider, model, base_url, api_key } behind a sealed Provider enum (OpenAiCompatible | Gemini | Mock). OpenAiCompatible (POST {base}/embeddings, bearer, {model, input, dimensions}) covers OpenRouter — the new default gateway — OpenAI direct, and self-hosted endpoints; Gemini keeps its RETRIEVAL_QUERY/RETRIEVAL_DOCUMENT task types; Mock is offline/deterministic. EmbedRole replaces the task-type string.

from_env() resolves provider via OMNIGRAPH_EMBED_PROVIDER (default openai-compatible), base/model via OMNIGRAPH_EMBED_BASE_URL/_MODEL, and the api key from OPENROUTER_API_KEY/OPENAI_API_KEY or GEMINI_API_KEY. BREAKING (pre-release, no back-compat): the default provider is now OpenRouter, OMNIGRAPH_GEMINI_BASE_URL is dropped, and Gemini-only users set OMNIGRAPH_EMBED_PROVIDER=gemini.

Folds in RFC-012 Phase 1 NFR floor: a total-operation OMNIGRAPH_EMBED_QUERY_DEADLINE_MS deadline (default 60s; 0=unbounded) bounds the ~121s worst case, and tracing spans (target omnigraph::embedding) record provider/model/dim/attempt/elapsed/outcome. The offline 'omnigraph embed' CLI follows the resolved provider (its hardcoded gemini-only bail removed). 17 engine embedding unit tests, 4 CLI embed tests, and the search integration suite (22) pass.

Cross-query client reuse and the docs refresh land in follow-up commits.

crates/omnigraph-cli/tests/system_local.rs

@@ -1111,6 +1111,11 @@ query vector_search($q: String) {
 
     let result = parse_stdout_json(&output_success(
         cli()
+            // Stored vectors above were produced with gemini-embedding-2-preview;
+            // pin the query-time embedder to the same provider/model so the
+            // auto-embedded `$q` lands in the same vector space.
+            .env("OMNIGRAPH_EMBED_PROVIDER", "gemini")
+            .env("OMNIGRAPH_EMBED_MODEL", "gemini-embedding-2-preview")
             .arg("read")
             .arg(&graph)
             .arg("--query")