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/process.rs

@@ -90,6 +90,27 @@ pub fn stop_pid(pid: u32, force: bool, timeout_secs: u64) -> bool {
     !is_running(pid)
 }
 
+/// Kill a process and its whole tree (process group on unix, taskkill /T on
+/// Windows). Used by the plugin runner and the lab benchmark to stop
+/// runaway scripts and timed-out task runs.
+pub fn kill_tree(pid: u32) {
+    #[cfg(unix)]
+    {
+        use std::os::unix::process::CommandExt;
+        let _ = Command::new("kill")
+            .arg("-KILL")
+            .arg(format!("-{pid}")) // negative pid = the process group
+            .status();
+        let _ = Command::new("kill").arg("-KILL").arg(pid.to_string()).status();
+    }
+    #[cfg(windows)]
+    {
+        let _ = Command::new("taskkill")
+            .args(["/PID", &pid.to_string(), "/T", "/F"])
+            .status();
+    }
+}
+
 /// Start a program detached: stdout/stderr are redirected to the agent log
 /// file. Returns the PID. Fails when the process exits immediately.
 pub fn spawn_background(