feat(manifest): un plugin peut contribuer des commandes slash via callback — #165 (QA verte)

Étend le manifeste et la validation SDK : un plugin déclare des commandes
slash (contributes.slashCommands) adossées à une callback (command id
enregistré via ctx.commands.registerCommand). Métadonnées UI exposées :
name, shortDescription, requiresConfirmation, when. Exemple hello-plugin
mis à jour avec une commande /hello.

Ces commandes transitent ensuite par le registry/contrat unifié (#162).

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

docs/commands-and-feedback.md

@@ -5,14 +5,20 @@ background work.
 
 ## Contract
 
-Every human menu click follows this sequence:
+Every menu click or plugin slash command follows this sequence:
 
 ```text
-menu item -> command id -> registered command handler -> optional task -> feedback surfaces
+manifest contribution -> command id -> registered command handler -> optional task -> feedback surfaces
 ```
 
 - A manifest menu item declares a `command` id; it does not run tools directly.
+- A manifest slash command declares a slash `name`, autocomplete metadata and a
+  `command` id; it does not run tools directly.
+- Menu items and slash commands may share the same `command` id, or point to
+  different handlers. The plugin owns that choice.
 - The command handler is the only place that decides whether work should start.
+- The host slash-command registry only lists/filters metadata and returns a
+  callback dispatch effect. The plugin handler decides what the command does.
 - A launched process is represented by a background task returned from
   `ctx.services.tasks.runCommand()`.
 - A skipped command is represented by the command handler return value and logs,