fix(agent): use provider registry for claw402, echo reasoning_content for thinking models, add Beta badge

- Agent now uses mcp.NewAIClientByProvider() for claw402 provider, ensuring
  x402 payment signing works correctly instead of generic HTTP client
- Added ReasoningContent field to Message/LLMResponse structs and wired
  serialization/parsing so DeepSeek thinking models work in multi-turn
- Added Beta badge to Agent nav tab in HeaderBar

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

web/src/components/common/HeaderBar.tsx

@@ -115,6 +115,7 @@ export default function HeaderBar({
                   page: 'agent',
                   path: ROUTES.agent,
                   label: 'Agent',
+                  badge: 'Beta',
                   requiresAuth: false,
                 },
                 {