fix: handle rate_limit_event crash in chat sessions

The Claude CLI sends `rate_limit_event` messages that the SDK's
`parse_message()` doesn't recognize, raising `MessageParseError` and
crashing all three chat session types (spec, assistant, expand).

Changes:
- Bump claude-agent-sdk minimum from 0.1.0 to 0.1.39
- Add `check_rate_limit_error()` helper in chat_constants.py that
  detects rate limits from both MessageParseError data payloads and
  error message text patterns
- Wrap `receive_response()` loops in all three `_query_claude()` methods
  with retry-on-rate-limit logic (up to 3 retries with backoff)
- Gracefully log and skip non-rate-limit MessageParseError instead of
  crashing the session
- Add `rate_limited` message type to frontend TypeScript types and
  handle it in useSpecChat, useAssistantChat, useExpandChat hooks to
  show "Rate limited. Retrying in Xs..." system messages

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

ui/src/hooks/useExpandChat.ts

@@ -226,6 +226,20 @@ export function useExpandChat({
             break
           }
 
+          case 'rate_limited': {
+            // Show rate limit info as system message
+            setMessages((prev) => [
+              ...prev,
+              {
+                id: generateId(),
+                role: 'system',
+                content: `Rate limited. Retrying in ${data.retry_in}s... (attempt ${data.attempt}/${data.max_attempts})`,
+                timestamp: new Date(),
+              },
+            ])
+            break
+          }
+
           case 'pong': {
             // Keep-alive response, nothing to do
             break