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/config.rs

@@ -136,6 +136,22 @@ pub struct Settings {
     /// Push automatically when the REPL exits (issue #66, opt-in).
     #[serde(default)]
     pub sync_on_exit: Option<bool>,
+    /// Plugin scripts (issue #75): default timeout and enable list.
+    #[serde(default)]
+    pub plugins: Option<PluginSettings>,
+}
+
+/// Plugin scripts settings (issue #75): how long one plugin may run and
+/// which plugins of the directory are enabled (empty = every plugin).
+#[derive(Debug, Clone, Serialize, Deserialize, Default)]
+#[serde(deny_unknown_fields)]
+pub struct PluginSettings {
+    /// Default timeout in seconds for every plugin (default 10).
+    #[serde(default)]
+    pub timeout_secs: Option<u64>,
+    /// Only run these plugin names (empty = all the plugins of the dir).
+    #[serde(default)]
+    pub enabled: Option<Vec<String>>,
 }
 
 /// Token price model: USD per million tokens (issue #49).
@@ -750,6 +766,9 @@ pub fn merge(base: &mut Config, overlay: Config) {
     if o.sync_on_exit.is_some() {
         s.sync_on_exit = o.sync_on_exit;
     }
+    if o.plugins.is_some() {
+        s.plugins = o.plugins;
+    }
     for (k, v) in o.hooks {
         s.hooks.insert(k, v);
     }