feat(session): persist model tier overrides per session

Store per-session config in SQLite and route /model and /reset through command fast-paths so channel sessions keep independent model selection across reconnects and restarts.

src/gateway/handlers/agent.test.ts

@@ -86,6 +86,27 @@ describe('createAgentHandlers command fast-path', () => {
     expect(((sent[0] as GatewayEvent).data as { content: string }).content).toContain('Session reset.');
   });
 
+  it('handles /model command via fast-path and persists session tier', async () => {
+    const sent: OutboundMessage[] = [];
+    const send = vi.fn((msg: OutboundMessage) => sent.push(msg));
+    const req: GatewayRequest = {
+      id: 4,
+      method: 'agent.send',
+      params: {
+        message: '/model fast',
+        connectionId: 'conn-1',
+        metadata: { isCommand: true, command: 'model', commandArgs: 'fast' },
+      },
+    };
+
+    await handlers['agent.send'](req, send);
+
+    expect(mockAgent.setModelTier).toHaveBeenCalledWith('fast');
+    expect(sessionManager.setSessionConfig).toHaveBeenCalledWith('ws', 'ws:conn-1', 'modelTier', 'fast');
+    expect(mockAgent.process).not.toHaveBeenCalled();
+    expect(((sent[0] as GatewayEvent).data as { content: string }).content).toContain('Switched to model tier: fast');
+  });
+
   it('falls through to agent.process for unknown commands', async () => {
     const sent: OutboundMessage[] = [];
     const send = vi.fn((msg: OutboundMessage) => sent.push(msg));