feat(agent): UI hot-swap de profil (A2) — commande Tauri + sélecteur + dialog

- Tauri : commande change_agent_profile + ChangeAgentProfileRequestDto/Dto
  (camelCase, relaunchedSession omis si None), câblage state.rs par composition.
- Front : gateway changeAgentProfile (adapters Tauri+mock), sélecteur de profil
  par agent, dialog de confirmation FR (« Changer le moteur abandonne l'historique
  de conversation… »), refresh sur event agentProfileChanged.

Tests : app-tauri dto 5/5 ; vitest 314/314 (mapping, payload, dialog gating,
libellé FR, refresh). 0 régression.

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

frontend/src/adapters/mock/index.ts

@@ -32,6 +32,7 @@ import type {
   Skill,
   SkillScope,
   Template,
+  TerminalSession,
   Unsubscribe,
 } from "@/domain";
 import type {
@@ -305,6 +306,61 @@ export class MockAgentGateway implements AgentGateway {
     this.contexts.delete(this.contextKey(projectId, agentId));
   }
 
+  async changeAgentProfile(
+    projectId: string,
+    agentId: string,
+    profileId: string,
+    rows: number,
+    cols: number,
+  ): Promise<{ agent: Agent; relaunchedSession?: TerminalSession }> {
+    const list = this.getAgents(projectId);
+    const idx = list.findIndex((a) => a.id === agentId);
+    if (idx === -1) {
+      const err: GatewayError = {
+        code: "NOT_FOUND",
+        message: `agent ${agentId} not found in project ${projectId}`,
+      };
+      throw err;
+    }
+    // Mutate the agent's runtime profile in place (the rest of the record —
+    // origin, synchronized, skills — is untouched).
+    list[idx] = { ...list[idx], profileId };
+    const agent = structuredClone(list[idx]);
+
+    // Hot-swap path: when the agent owns a live session, the engine restarts —
+    // the old PTY is killed (history abandoned, per the product decision) and a
+    // fresh session is minted in the same cell, surfaced for the caller to
+    // rebind. No live session ⇒ nothing to relaunch.
+    const nodeId = this.liveByAgent.get(agentId);
+    const oldSessionId = this.liveSessionByAgent.get(agentId);
+    if (!nodeId || !oldSessionId) {
+      return { agent };
+    }
+    // Kill the old session.
+    const old = this.sessions.get(oldSessionId);
+    if (old) old.closed = true;
+    this.sessions.delete(oldSessionId);
+
+    // Mint a fresh, detached session (no view sink yet; the cell reattaches).
+    this.sessionSeq += 1;
+    const sessionId = `mock-agent-session-${this.sessionSeq}`;
+    const session = new MockPtySession(sessionId, () => {});
+    session.detach();
+    this.sessions.set(sessionId, session);
+    this.liveSessionByAgent.set(agentId, sessionId);
+    this.liveByAgent.set(agentId, nodeId);
+
+    return {
+      agent,
+      relaunchedSession: {
+        sessionId,
+        cwd: `/home/user/mock-project`,
+        rows,
+        cols,
+      },
+    };
+  }
+
   // ── Internal helpers for MockTemplateGateway (same-package use only) ──
 
   /**