Route Store through agent-store substrate (dogfood the causal store)

Cargo.toml

@@ -21,6 +21,9 @@ repository = "https://github.com/hartsock/modulex-mcp"
 [workspace.dependencies]
 modulex-core = { path = "crates/modulex-core", version = "=0.1.0" }
 
+# agent-store: causal-store substrate (Generation counter, SqliteBackend).
+agent-store = "0.1"
+
 agent-bridle-core = "0.1.0"
 agent-bridle-tool-web = "0.1.0"
 anyhow = "1.0"

crates/modulex-core/Cargo.toml

@@ -20,6 +20,7 @@ test-support = []
 [dependencies]
 agent-bridle-core = { workspace = true }
 agent-bridle-tool-web = { workspace = true, optional = true }
+agent-store = { workspace = true }
 anyhow = { workspace = true }
 blake3 = { workspace = true, optional = true }
 rusqlite = { workspace = true }

crates/modulex-core/src/config.rs

@@ -184,12 +184,17 @@ pub struct McpConfig {
     pub deny: Vec<String>,
 }
 
-/// Agent state store location (`[store]`).
+/// Agent state store config (`[store]`).
 #[derive(Clone, Debug, Default, Deserialize)]
 pub struct StoreConfig {
     /// SQLite path; empty = `$MODULEX_STORE` → `~/.modulex/store.db`.
     #[serde(default)]
     pub path: String,
+    /// Backend-selection policy — a flat knob, `[store] backend = "sqlite"`.
+    /// Defaults to the safe, daemonless SQLite backend; change direction
+    /// (Postgres, later) by editing config, not code.
+    #[serde(flatten)]
+    pub policy: agent_store::StorePolicy,
 }
 
 /// A fixed date to count down to.
@@ -367,6 +372,26 @@ pub fn expand_tilde(path: &str) -> PathBuf {
 mod tests {
     use super::*;
 
+    #[test]
+    fn store_backend_is_a_flat_policy_knob() {
+        // The agent-store StorePolicy flattens into [store] as a flat knob,
+        // alongside `path`, parsed via the real Config path.
+        let cfg = Config::from_toml(
+            r#"
+[store]
+path = "/tmp/x.db"
+backend = "postgres"
+"#,
+        )
+        .unwrap();
+        assert_eq!(cfg.store.path, "/tmp/x.db");
+        assert_eq!(cfg.store.policy.backend, agent_store::BackendKind::Postgres);
+
+        // Default: no backend key => the safe SQLite default.
+        let cfg = Config::from_toml("[store]\npath = \"/tmp/y.db\"\n").unwrap();
+        assert_eq!(cfg.store.policy.backend, agent_store::BackendKind::Sqlite);
+    }
+
     #[test]
     fn minimal_routine_parses_with_flattened_params() {
         let cfg = Config::from_toml(

crates/modulex-core/src/engine.rs

@@ -103,7 +103,7 @@ impl Engine {
             (!config.store.path.is_empty()).then_some(config.store.path.as_str()),
             home.as_deref(),
         );
-        let store = match crate::store::Store::open(&path) {
+        let store = match crate::store::Store::open_with_policy(&config.store.policy, &path) {
             Ok(store) => Some(Arc::new(store)),
             Err(e) => {
                 eprintln!(