docs(sdk): éclate et étend la documentation SDK par sujet

Remplace le README monolithique par un point d'entrée vers des pages dédiées
(manifest, activation/contexte, menus, commandes/feedback, layouts React,
fenêtres, services, packaging/distribution) pour couvrir #142-#144 et
faciliter la navigation.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/layouts-react.md

@@ -0,0 +1,85 @@
+# Layouts With React
+
+Plugin layouts are real React components. They can use JSX and hooks.
+
+```tsx
+import type { PluginLayoutProps } from "@idea/plugin-sdk";
+import { useState } from "react";
+
+export function Dashboard(props: PluginLayoutProps): React.ReactElement {
+  const [localClicks, setLocalClicks] = useState(0);
+
+  return (
+    <button
+      type="button"
+      onClick={() => {
+        setLocalClicks((value) => value + 1);
+        props.setState({ localClicks: localClicks + 1 });
+      }}
+    >
+      Clicked {localClicks}
+    </button>
+  );
+}
+```
+
+Register the component for a manifest-declared layout type:
+
+```ts
+ctx.layouts?.register({
+  type: "hello-plugin.dashboard",
+  component: Dashboard
+});
+```
+
+## Props
+
+- `projectId`: project hosting the layout.
+- `nodeId`: stable host node id for this layout instance.
+- `layoutType`: manifest layout `type`.
+- `state`: JSON-serializable host-persisted state.
+- `setState(next)`: replace host-persisted state.
+- `availability`: currently `"available"` while mounted.
+
+Call `setState` from user actions, effects or async callbacks. Do not call it
+unconditionally during render.
+
+## Host React
+
+Plugins must import React normally:
+
+```ts
+import { useEffect, useMemo, useState } from "react";
+```
+
+At runtime IdeA resolves these bare imports to the host instance:
+
+- `react`
+- `react-dom`
+- `react-dom/client`
+- `react/jsx-runtime`
+- `react/jsx-dev-runtime`
+
+Do not bundle your own copy of React or ReactDOM into the plugin. Declare them
+as `peerDependencies` and `devDependencies` for local typechecking/builds.
+
+```json
+{
+  "peerDependencies": {
+    "react": "^18.2.0 || ^19.0.0",
+    "react-dom": "^18.2.0 || ^19.0.0"
+  },
+  "devDependencies": {
+    "@types/react": "^19.0.0",
+    "@types/react-dom": "^19.0.0",
+    "react": "^19.0.0",
+    "react-dom": "^19.0.0"
+  }
+}
+```
+
+## Build
+
+Use `jsx: "react-jsx"` in `tsconfig`. A plain multi-file ESM `tsc` build is
+supported as long as every emitted file imported by `main` is included in the
+archive.