feat: phase 2 — am lab (benchmark N agents) + plugins d'événements (contrat JSON)

- #62: am lab --agents a,b --task <f> — séquentiel ou --parallel, capture
  durée/exit/sortie/coût estimé, rapport comparatif tableau + --json stable,
  tâches versionnables dans <state>/lab/ (--list), rejouables à l'identique
- #75: plugins/ sous state_dir, déclenchés sur on_install/on_start/on_stop/
  on_update (App::emit), contrat JSON stdin/stdout, timeout configurable,
  échec non bloquant, garde anti-récursion AM_PLUGINS_RUNNING, am plugins
  + am plugins --test <name> (CI), exemples notify + ci-webhook
- intégration REPL (parse, complétion, bannière, is_am_command), help, tips,
  man pages (am-lab.1, am-plugins.1), README, ROADMAP (26/27), v0.5.4
- 334 tests verts (309 + 25)

closes #62
closes #75

src/app.rs

@@ -131,11 +131,13 @@ impl App {
     }
 
     /// Append an event to the journal; failures are never fatal (logged at
-    /// verbose level only).
+    /// verbose level only). The plugins subscribed to the event kind are
+    /// then fired (issue #75) — non-blocking, never fatal either.
     pub fn emit(&self, event: &crate::events::Event) {
         if let Err(e) = crate::events::append(&self.events_dir(), event) {
             self.log.verbose(&format!("cannot write event journal: {e:#}"));
         }
+        crate::plugins::dispatch(self, event);
     }
 
     /// Ask the user for confirmation on stderr. Accepts y/yes/o/oui.