fix(runtime): stop/restart d'agent visible robuste — kill réel, état stopped, relance froide (#196, QA verte)

Solidifie le cycle stop/restart d'un agent visible en cellule:

- terminal/registry: kill effectif de la TUI native et de la CLI custom, arrêt idempotent du live runtime (PTY + structuré), distinction nette « agent stoppé » vs « pas de session ».

- workstate + lifecycle: projection d'un état runtime explicite « stopped » (AgentRuntimeState: running/stopping/stopped/failedStart), réconcilié avec le live state; traitement des courses « agent not found ».

- relance froide via freshStart: repart de zéro sans réattacher une session morte et recharge la configuration courante de l'agent (y compris changement de modèle).

- contrat projeté côté adapters frontend (ports/domain/normalization/mock) pour la consommation UI future (#197/#198), sans surface UI ici.

Tests QA verts: workstate_actions, orchestrator_service stop_agent, agent_lifecycle (reopen/fresh_start), workstate stopped runtime state, vitest adapter stop_live_agent.

frontend/src/ports/index.ts

@@ -378,6 +378,8 @@ export interface OpenTerminalOptions {
    * meaningful for {@link AgentGateway.launchAgent}; ignored for plain terminals.
    */
   conversationId?: string;
+  /** Ignore a previous conversation id and request a cold, clean launch. */
+  freshStart?: boolean;
   /**
    * The layout leaf (node) hosting this launch. Drives the "one live session per
    * agent" invariant backend-side: launching an agent already running in a
@@ -400,6 +402,8 @@ export interface LiveAgent {
   sessionId: string;
   /** Runtime kind backing the live agent. */
   kind: "pty" | "structured";
+  /** Runtime lifecycle state. Live agents are always running. */
+  runtimeState?: AgentRuntimeState;
 }
 
 /** Result returned when the backend stops a live agent. */
@@ -407,8 +411,15 @@ export interface StoppedLiveAgent {
   agentId: string;
   sessionId: string;
   kind: "pty" | "structured";
+  runtimeState?: AgentRuntimeState;
 }
 
+export type AgentRuntimeState =
+  | "running"
+  | "stopping"
+  | "stopped"
+  | "failedStart";
+
 /**
  * A live terminal handle returned by {@link TerminalGateway.openTerminal}.
  *