chore: document architecture gaps and add dev hot reload

- add npm dev:hot wrapper using cargo-watch and page reload polling

- add mnote-web dev hot reload endpoint and coverage

- record current architecture review and tracked bug findings across realtime, tree, editor, and AI runtimes

Verification:

- node scripts/task-dev-hot-plan-test.js

- node --check scripts/dev-hot.js

- cargo test -p mnote-web dev_hot -- --nocapture

rust/crates/mnote-web/src/routes/mod.rs

@@ -1,6 +1,7 @@
 mod bridge;
 pub(crate) mod command_support;
 mod compat;
+mod dev_hot;
 mod documents;
 mod editor;
 mod gateway;
@@ -80,6 +81,7 @@ pub fn build_router(state: AppState) -> Router {
         )
         .route("/api/search/documents", post(search::documents))
         .route("/api/gateway/health", get(gateway::gateway_health))
+        .route("/api/dev/hot-reload", get(dev_hot::hot_reload))
         .route("/api/runtime/config", get(session::runtime_config))
         .route("/api/auth", post(gateway::auth_api))
         .route("/api/auth/session", get(session::session))
@@ -350,4 +352,18 @@ mod tests {
             );
         }
     }
+
+    #[tokio::test]
+    async fn dev_hot_reload_endpoint_is_mounted() {
+        let response = app(false)
+            .oneshot(
+                Request::builder()
+                    .uri("/api/dev/hot-reload")
+                    .body(Body::empty())
+                    .expect("request"),
+            )
+            .await
+            .expect("response");
+        assert_eq!(response.status(), StatusCode::OK);
+    }
 }