fix(agents): enforcer l'invariant « 1 session vivante par agent » (singleton)

Un agent ne peut tourner que dans une seule cellule à la fois. La garde dans
LaunchAgent refuse le spawn si l'agent est déjà vivant dans un autre node
(AGENT_ALREADY_RUNNING) ; idempotent sur le même node ; le chemin resume
(agent mort) reste inchangé. Le node_id est désormais plombé jusqu'au use case.

Corrige le reset asymétrique d'une cellule au changement d'onglet : deux leaves
partageant le même agent id rendaient session_for_agent/is_agent_live/stop_agent
ambigus (cible arbitraire). Le churn reset/reattach déclenchait aussi les accents
mélangés (FIFO intact, non touché).

- snapshot agentWasRunning calculé par node (is_node_live) et non par agent
- commande list_live_agents + live_agents()/node_for_agent()/is_node_live()
- UI : dropdown grise les agents déjà placés ailleurs ; 2e cellule en doublon
  affiche « disponible » au lieu d'une relance fantôme

Tests : cargo test (application + app-tauri) vert ; tsc + vitest vert.

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

frontend/src/adapters/mock/index.ts

@@ -40,6 +40,7 @@ import type {
   CreateSkillInput,
   CreateTemplateInput,
   Gateways,
+  LiveAgent,
   MemoryGateway,
   GitGateway,
   LayoutGateway,
@@ -170,6 +171,13 @@ export class MockAgentGateway implements AgentGateway {
   private sessionSeq = 0;
   /** Live agent PTY sessions, kept across detach so reattach can find them. */
   private sessions = new Map<string, MockPtySession>();
+  /**
+   * The cell each live agent currently runs in (`agentId → nodeId`). Mirrors the
+   * backend "one live session per agent" invariant: launching an agent already
+   * present here in a *different* node is refused; closing its session clears it.
+   * Only populated when the caller supplies a `nodeId`.
+   */
+  private liveByAgent = new Map<string, string>();
 
   private getAgents(projectId: string): Agent[] {
     if (!this.agents.has(projectId)) this.agents.set(projectId, []);
@@ -184,6 +192,15 @@ export class MockAgentGateway implements AgentGateway {
     return structuredClone(this.getAgents(projectId));
   }
 
+  async listLiveAgents(projectId: string): Promise<LiveAgent[]> {
+    // Only report agents that belong to this project (ids are disjoint across
+    // projects in the mock, mirroring the backend's per-project agent ids).
+    const known = new Set(this.getAgents(projectId).map((a) => a.id));
+    return [...this.liveByAgent.entries()]
+      .filter(([agentId]) => known.has(agentId))
+      .map(([agentId, nodeId]) => ({ agentId, nodeId }));
+  }
+
   async createAgent(projectId: string, input: CreateAgentInput): Promise<Agent> {
     const list = this.getAgents(projectId);
     const slug = slugify(input.name) || "agent";
@@ -320,17 +337,36 @@ export class MockAgentGateway implements AgentGateway {
       };
       throw err;
     }
+    // Singleton invariant: refuse a launch when the agent is already live in a
+    // *different* cell (mirrors the backend `AGENT_ALREADY_RUNNING`). The same
+    // node is allowed (idempotent re-launch of the very same cell).
+    const liveNode = this.liveByAgent.get(agentId);
+    if (liveNode !== undefined && options.nodeId && liveNode !== options.nodeId) {
+      const err: GatewayError = {
+        code: "AGENT_ALREADY_RUNNING",
+        message: `agent ${agentId} is already running in cell ${liveNode}`,
+      };
+      throw err;
+    }
     this.sessionSeq += 1;
     const sessionId = `mock-agent-session-${this.sessionSeq}`;
     const cwd = options.cwd;
     const enc = new TextEncoder();
     const session = new MockPtySession(sessionId, onData);
     this.sessions.set(sessionId, session);
+    // Record liveness for `listLiveAgents` + the guard above (only when the
+    // caller pins a node).
+    if (options.nodeId) this.liveByAgent.set(agentId, options.nodeId);
     // Greet so something is visible immediately (mirrors MockTerminalGateway).
     queueMicrotask(() =>
       session.emit(enc.encode(`agent ${agentId} @ ${cwd}\r\n`)),
     );
-    const handle = makeMockHandle(session, () => this.sessions.delete(sessionId));
+    const handle = makeMockHandle(session, () => {
+      this.sessions.delete(sessionId);
+      if (this.liveByAgent.get(agentId) === options.nodeId) {
+        this.liveByAgent.delete(agentId);
+      }
+    });
     // Simulate session assignment (T4b): a fresh cell (no conversation id) gets a
     // newly-minted id surfaced on the handle so the caller persists it; a cell
     // that already carries an id resumes it and nothing new is assigned.