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/dev_hot.rs

@@ -0,0 +1,51 @@
+use axum::http::{HeaderMap, HeaderValue};
+use axum::response::IntoResponse;
+use axum::Json;
+use serde::Serialize;
+use std::sync::LazyLock;
+use std::time::{SystemTime, UNIX_EPOCH};
+
+const HEADER_MNOTE_WEB_OWNER: &str = "x-mnote-web-owner";
+
+static DEV_HOT_BOOT_ID: LazyLock<String> = LazyLock::new(|| {
+    let now = SystemTime::now()
+        .duration_since(UNIX_EPOCH)
+        .map(|duration| duration.as_millis())
+        .unwrap_or_default();
+    format!("{}-{now}", std::process::id())
+});
+
+#[derive(Debug, Serialize)]
+#[serde(rename_all = "camelCase")]
+pub struct DevHotReloadPayload {
+    ok: bool,
+    enabled: bool,
+    boot_id: &'static str,
+}
+
+pub fn dev_hot_reload_enabled() -> bool {
+    matches!(
+        std::env::var("MNOTE_WEB_DEV_HOT_RELOAD")
+            .unwrap_or_default()
+            .trim()
+            .to_ascii_lowercase()
+            .as_str(),
+        "1" | "true" | "yes" | "on"
+    )
+}
+
+pub async fn hot_reload() -> impl IntoResponse {
+    let mut headers = HeaderMap::new();
+    headers.insert(
+        HEADER_MNOTE_WEB_OWNER,
+        HeaderValue::from_static("mnote-web"),
+    );
+    (
+        headers,
+        Json(DevHotReloadPayload {
+            ok: true,
+            enabled: dev_hot_reload_enabled(),
+            boot_id: DEV_HOT_BOOT_ID.as_str(),
+        }),
+    )
+}