fix: code review fixes from gt-2tp and gt-y0t

- Replace custom contains() with strings.Contains in init.go
- Make session stop --force flag actually work
- Update session.Manager.Stop() to accept force parameter

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/session/manager_test.go

@@ -105,7 +105,7 @@ func TestStopNotFound(t *testing.T) {
 	}
 	m := NewManager(tmux.NewTmux(), r)
 
-	err := m.Stop("Toast")
+	err := m.Stop("Toast", false)
 	if err != ErrSessionNotFound {
 		t.Errorf("Stop = %v, want ErrSessionNotFound", err)
 	}