feat: let workflow rag nodes select a RAG driver

`RagNode` gains an optional `driver`, forwarded into `RagInitConfig` so a
graph node can build its knowledge base on duckdb instead of yaml. Nodes
that name no driver forward `None`, which still resolves to yaml, so
existing workflows are unaffected.

An unknown driver is rejected up front rather than at construction time.
`Rag::create` dispatches unknown drivers to its yaml catch-all, so a typo
would otherwise embed every document and persist the bogus string, after
which every subsequent load fails validation and the agent cannot start.
The check asks `RagData::validate()` through a probe value instead of
restating the list of valid drivers, so the two cannot drift.

src/config/agent.rs

@@ -12,6 +12,7 @@ use crate::config::prompts::{
     DEFAULT_SPAWN_INSTRUCTIONS, DEFAULT_TEAMMATE_INSTRUCTIONS, DEFAULT_TODO_INSTRUCTIONS,
     DEFAULT_USER_INTERACTION_INSTRUCTIONS,
 };
+use crate::graph::types::RagNode;
 use crate::graph::{Graph, GraphParser, NodeType};
 use crate::rag::RagInitConfig;
 use crate::vault::SECRET_RE;
@@ -952,6 +953,30 @@ fn resolve_document_paths(
     Ok(document_paths)
 }
 
+/// How a graph rag node describes the knowledge base it wants built.
+///
+/// `driver` is forwarded as-is: `None` means the node did not ask for one, which
+/// `RagInitConfig` resolves to yaml, so workflows written before drivers existed
+/// keep their current storage.
+///
+/// Every field is now named explicitly, so adding one to `RagInitConfig` breaks
+/// this literal. That is deliberate: the new field then gets a decision about
+/// whether a rag node can drive it, instead of silently taking its default.
+fn rag_init_config(rag_node: &RagNode) -> RagInitConfig {
+    RagInitConfig {
+        embedding_model: rag_node.embedding_model.clone(),
+        chunk_size: rag_node.chunk_size,
+        chunk_overlap: rag_node.chunk_overlap,
+        reranker_model: rag_node.reranker_model.clone(),
+        top_k: rag_node.top_k,
+        batch_size: rag_node.batch_size,
+        extractor_model: rag_node.extractor_model.clone(),
+        extractor_prompt: rag_node.extractor_prompt.clone(),
+        graph_hops: rag_node.graph_hops,
+        driver: rag_node.driver.clone(),
+    }
+}
+
 #[allow(clippy::too_many_arguments)]
 async fn init_graph_rags(
     app: &AppConfig,
@@ -989,21 +1014,18 @@ async fn init_graph_rags(
                 })
                 .await?
         } else {
-            let config = RagInitConfig {
-                embedding_model: rag_node.embedding_model.clone(),
-                chunk_size: rag_node.chunk_size,
-                chunk_overlap: rag_node.chunk_overlap,
-                reranker_model: rag_node.reranker_model.clone(),
-                top_k: rag_node.top_k,
-                batch_size: rag_node.batch_size,
-                extractor_model: rag_node.extractor_model.clone(),
-                extractor_prompt: rag_node.extractor_prompt.clone(),
-                graph_hops: rag_node.graph_hops,
-                // Graph-node RAGs are yaml-only: `RagNode` has no `driver` field, so
-                // there is nothing to forward. The rest-pattern also keeps this literal
-                // from breaking on future `RagInitConfig` additions.
-                ..Default::default()
-            };
+            // Checked before anything is built: an unknown driver would otherwise
+            // fall through `Rag::create`'s catch-all to a yaml store, embed every
+            // document, and persist the bogus driver string. The RAG would then be
+            // rejected on every subsequent load, leaving the agent unstartable.
+            // Graph validation catches this too, but it is skipped when
+            // `validate_before_run` is off, so this guard is the load-bearing one.
+            if let Some(driver) = &rag_node.driver
+                && let Some(message) = crate::graph::validator::rag_driver_error(driver)
+            {
+                bail!("rag node '{node_id}': {message}");
+            }
+            let config = rag_init_config(rag_node);
             let fully_specified = config.embedding_model.is_some()
                 && config.chunk_size.is_some()
                 && config.chunk_overlap.is_some();
@@ -1337,4 +1359,39 @@ version: "1.0"
 
         assert_eq!(meta.description, "");
     }
+
+    #[test]
+    fn rag_init_config_forwards_an_explicit_driver() {
+        let node: RagNode =
+            serde_yaml::from_str("documents: [\"./docs\"]\ndriver: duckdb\n").unwrap();
+
+        assert_eq!(rag_init_config(&node).driver.as_deref(), Some("duckdb"));
+    }
+
+    /// A node that names no driver must forward `None`, which `RagInitConfig`
+    /// documents as "yaml". Existing workflows therefore keep their yaml store.
+    #[test]
+    fn rag_init_config_leaves_the_driver_unset_by_default() {
+        let node: RagNode = serde_yaml::from_str("documents: [\"./docs\"]\n").unwrap();
+
+        assert_eq!(rag_init_config(&node).driver, None);
+    }
+
+    /// The driver must ride alongside the rest of the node's settings, not
+    /// replace them.
+    #[test]
+    fn rag_init_config_forwards_the_other_settings_too() {
+        let node: RagNode = serde_yaml::from_str(
+            "documents: [\"./docs\"]\ndriver: duckdb\nchunk_size: 512\nchunk_overlap: 64\ntop_k: 7\nembedding_model: some:model\n",
+        )
+        .unwrap();
+
+        let config = rag_init_config(&node);
+
+        assert_eq!(config.driver.as_deref(), Some("duckdb"));
+        assert_eq!(config.chunk_size, Some(512));
+        assert_eq!(config.chunk_overlap, Some(64));
+        assert_eq!(config.top_k, Some(7));
+        assert_eq!(config.embedding_model.as_deref(), Some("some:model"));
+    }
 }