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/activation-context.md

@@ -0,0 +1,60 @@
+# Activation And Context
+
+The plugin entrypoint exports `activate(ctx)`.
+
+```ts
+import type { ActivateContext, IdeAPluginModule } from "@idea/plugin-sdk";
+
+export function activate(ctx: ActivateContext): void {
+  ctx.logger.info("activated", { pluginId: ctx.pluginId });
+}
+
+export default { activate } satisfies IdeAPluginModule;
+```
+
+IdeA accepts either a named `activate` export or a default export containing
+`activate`.
+
+## Activation Scope
+
+`activationScope` defaults to `"app"`. App-scoped plugins activate during app
+bootstrap. Project-scoped plugins are kept pending until a project is focused,
+then activated once for the app session.
+
+Choose `"project"` only when `activate(ctx)` must immediately read project
+state. Menu commands and layouts can usually stay app-scoped and check for a
+focused project at invocation/render time.
+
+## Context Fields
+
+- `pluginId`, `pluginDisplayName`, `version`: host-provided identity.
+- `logger`: `debug`, `info`, `warn`, `error`.
+- `subscriptions`: push disposables returned by command/layout/watch
+  registrations.
+- `commands`: command registry for declared menu commands.
+- `layouts`: layout registry for declared layout types.
+- `menu`: marker for the plugin-owned menu surface.
+- `storage`: plugin-owned persistent key/value storage.
+- `services`: public host service facade for plugins declaring `ui` or `tooling`
+  capabilities.
+
+The context never exposes internal IdeA gateways or Tauri commands. Use
+`ctx.services` and the registration APIs instead.
+
+## Disposal
+
+Push every returned disposable to `ctx.subscriptions`:
+
+```ts
+const disposable = ctx.commands?.registerCommand("com.example.run", run);
+if (disposable) ctx.subscriptions.push(disposable);
+```
+
+IdeA disposes these handles best-effort when the plugin is unloaded or the app
+session ends.
+
+## Storage
+
+Use `ctx.storage` for plugin-owned counters, flags, preferences and small caches.
+Use workspace/config services only for project-owned files or configuration that
+the user expects to see in the project.