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/manifest.md

@@ -0,0 +1,68 @@
+# Manifest
+
+Every plugin package has an `idea-plugin.json` file at the archive root.
+
+```json
+{
+  "ideaPluginManifestVersion": 1,
+  "id": "com.example.hello-plugin",
+  "displayName": "Hello Plugin",
+  "publisher": "Example",
+  "version": "0.1.0",
+  "description": "Example IdeA plugin.",
+  "main": "dist/index.js",
+  "engines": {
+    "idea": ">=0.1.0"
+  },
+  "trustLevel": "full",
+  "capabilities": ["ui", "tooling"],
+  "activationScope": "app",
+  "contributes": {
+    "menus": [],
+    "menuItems": [],
+    "layouts": [],
+    "mcpServers": []
+  }
+}
+```
+
+## Required Fields
+
+- `ideaPluginManifestVersion`: currently `1`.
+- `id`: stable lowercase id using letters, digits, dots and dashes. It must start
+  and end with an alphanumeric character.
+- `displayName`: human-readable plugin name.
+- `version`: semver version.
+- `main`: package-relative ESM entrypoint loaded by IdeA.
+- `trustLevel`: currently `"full"`.
+
+## Optional Fields
+
+- `description`, `publisher`.
+- `engines.idea`: host compatibility hint.
+- `activationScope`: `"app"` by default, or `"project"` when activation needs a
+  focused project immediately.
+- `capabilities`: `"ui"`, `"tooling"` and/or `"mcp"`.
+
+## Contributions
+
+`contributes.layouts` is the only surface for plugin layouts. The same layout
+contribution can be mounted inside an IdeA layout cell or opened in a detached OS
+window. Do not add a separate manifest contribution type for windows.
+
+Contribution ids are part of the runtime contract:
+
+- commands can only register ids declared by `contributes.menuItems[*].command`;
+- layouts can only register `type` values declared by `contributes.layouts`;
+- `services.windows.open({ layoutType })` only accepts a layout type declared by
+  the calling plugin.
+
+## Validation
+
+Use the SDK validator in tests or build tooling:
+
+```ts
+import { assertPluginManifest } from "@idea/plugin-sdk";
+
+assertPluginManifest(JSON.parse(manifestText));
+```