fix(claude-cli): check scanner.Err() and increase stream buffer to 10MB (#96)

Fixes #94. The stream-json scanner never checked scanner.Err() after the
scan loop, so if a line exceeded the 1MB buffer limit the scanner would
stop with bufio.ErrTooLong and the response would be silently truncated.

- Check scanner.Err() after the loop and return an explicit error
- Increase max buffer from 1MB to 10MB to handle large tool outputs

Co-authored-by: Luvu182 <208665161+Luvu182@users.noreply.github.com>

internal/providers/claude_cli_chat.go

@@ -138,7 +138,7 @@ func (p *ClaudeCLIProvider) ChatStream(ctx context.Context, req ChatRequest, onC
 
 	// Parse stream-json line-by-line
 	scanner := bufio.NewScanner(stdout)
-	scanner.Buffer(make([]byte, 0, 256*1024), 1024*1024) // 256KB initial, 1MB max
+	scanner.Buffer(make([]byte, 0, 256*1024), 10*1024*1024) // 256KB initial, 10MB max
 
 	var finalResp ChatResponse
 	var contentBuf strings.Builder
@@ -194,6 +194,10 @@ func (p *ClaudeCLIProvider) ChatStream(ctx context.Context, req ChatRequest, onC
 		}
 	}
 
+	if err := scanner.Err(); err != nil {
+		return nil, fmt.Errorf("claude-cli: stream read error: %w", err)
+	}
+
 	if err := cmd.Wait(); err != nil {
 		if debugFile != nil {
 			fmt.Fprintf(debugFile, "\n=== STDERR:\n%s\n=== EXIT ERROR: %v\n", stderrBuf.String(), err)