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

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

@@ -0,0 +1,18 @@
+#!/usr/bin/env node
+"use strict";
+
+const assert = require("node:assert");
+const { buildDevHotEnv } = require("./dev-hot.js");
+
+const env = buildDevHotEnv({
+  FRONTEND_PORT: "3200",
+  MNOTE_WEB_CMD: "",
+});
+
+assert.equal(env.MNOTE_WEB_DEV_HOT_RELOAD, "1");
+assert.match(env.MNOTE_WEB_CMD, /cargo watch/);
+assert.match(env.MNOTE_WEB_CMD, /run -p mnote-web --bin mnote-web/);
+assert.match(env.MNOTE_WEB_CMD, /crates\/mnote-web\/src/);
+assert.equal(env.FRONTEND_PORT, "3200");
+
+console.log(JSON.stringify({ ok: true, command: env.MNOTE_WEB_CMD }, null, 2));