feat(FN-2699): switch runtime source selectors to tab controls

- Replace runtime source radio groups with tablist/tab buttons in NewAgentDialog and AgentDetailView
- Preserve runtime mode behavior by keeping model/runtime selection handlers and reset logic intact
- Align AgentDetailView runtime selector styling with tab pattern and remove obsolete radio-toggle CSS
- Update AgentDetailView tests to assert tab semantics via role and aria-selected

packages/dashboard/app/components/NewAgentDialog.css

@@ -150,58 +150,6 @@
   gap: var(--space-sm);
 }
 
-.agent-runtime-mode-toggle {
-  display: grid;
-  grid-template-columns: repeat(2, minmax(0, 1fr));
-  gap: var(--space-sm);
-}
-
-.agent-runtime-mode-option {
-  position: relative;
-  display: flex;
-}
-
-.agent-runtime-mode-option input {
-  position: absolute;
-  width: var(--space-xs);
-  height: var(--space-xs);
-  margin: 0;
-  opacity: 0;
-  pointer-events: none;
-}
-
-.agent-runtime-mode-option span {
-  width: 100%;
-  border: 1px solid var(--border);
-  border-radius: var(--radius-md);
-  background: var(--card);
-  color: var(--text-muted);
-  padding: var(--space-sm) var(--space-md);
-  text-align: center;
-  font-size: calc(var(--space-sm) + var(--space-xs));
-  font-weight: 600;
-  cursor: pointer;
-  transition: border-color var(--transition-fast), color var(--transition-fast), background var(--transition-fast), box-shadow var(--transition-fast);
-}
-
-.agent-runtime-mode-option input:checked + span {
-  border-color: var(--todo);
-  color: var(--text);
-  background: color-mix(in srgb, var(--todo) 12%, transparent);
-}
-
-.agent-runtime-mode-option input:focus-visible + span {
-  outline: none;
-  border-color: var(--todo);
-  box-shadow: var(--focus-ring-strong);
-}
-
-.agent-runtime-mode-option span:hover {
-  border-color: var(--todo);
-  color: var(--text);
-  background: var(--card-hover);
-}
-
 .agent-role-grid {
   display: grid;
   grid-template-columns: repeat(auto-fill, minmax(calc(var(--space-xl) * 3 + var(--space-sm)), 1fr));
@@ -537,9 +485,4 @@
   padding: var(--space-sm) 0;
 }
 
-@media (max-width: 768px) {
-  .agent-runtime-mode-toggle {
-    grid-template-columns: 1fr;
-  }
-}