refactor: move model info to constants

- Added EMBEDDING_MODEL and RERANKER_MODEL to `constants.ts`.
- Updated imports to use these constants in `agent.ts`, `agent_tools/reranker.ts`, and `setup/ingest.ts`.
- Replaced hard‑coded model strings with the new constants for Ollama embeddings and reranker.

agent.ts

@@ -11,10 +11,10 @@ import {
   createSectionRetrieverTool,
   createSpecRetrieverTool,
 } from "./agent_tools";
-import { GRAPH_FILE, STORAGE_DIR } from "./constants";
+import { EMBEDDING_MODEL, GRAPH_FILE, STORAGE_DIR } from "./constants";
 
 const embeddings = new OllamaEmbeddings({
-  model: "qwen3-embedding:0.6b",
+  model: EMBEDDING_MODEL,
 });
 
 const config = JSON.parse(fs.readFileSync("./config.json", "utf-8"));