feat: per-session store + configurable host limits; fix divider drag runaway

- state: side-store per conversation, injected as a prop instead of a module singleton
- host: schemastery Config (read/media/list/terminal limits) resolved from cordis config
- shape: types/exports/peerDependencies for the web profile + plugin-shape spec
- fix: split divider emits incremental deltas so drags track the cursor 1:1
- docs: README refresh

src/client/state.ts

@@ -624,5 +624,13 @@ export class SidebarStore {
   }
 }
 
-/** Module-level store instance (one per page; the plugin mounts once). */
-export const sidebarStore = new SidebarStore()
+/**
+ * Create one sidebar store instance. Production code calls this only from
+ * the client plugin's `apply` (the instance is handed to components as a
+ * prop); tests call it directly. No module-level singleton: the store's
+ * lifetime belongs to the plugin activation, exactly like the official
+ * `createXXXStore()` factory rule.
+ */
+export function createSidebarStore(): SidebarStore {
+  return new SidebarStore()
+}