Fix code fence closing without trailing newline

When a code block ended without a trailing newline after the closing
\`\`\`, two bugs occurred in flush_incomplete():

1. The closing \`\`\` was included as part of the code block content
   (displayed with syntax highlighting)
2. The same \`\`\` was then emitted again as literal text because
   current_line was not cleared after being pushed to block_buffer

The fix:
- Check if current_line is the closing fence before adding to block_buffer
- Always clear current_line after processing in the CodeBlock case

Added two tests:
- test_code_fence_after_blank_line: code fence with trailing newline
- test_code_fence_no_trailing_newline: code fence without trailing newline

crates/g3-cli/src/streaming_markdown.rs

@@ -697,7 +697,16 @@ impl StreamingMarkdownFormatter {
                 // Unclosed code block - emit as-is
                 if !self.block_buffer.is_empty() || !self.current_line.is_empty() {
                     if !self.current_line.is_empty() {
-                        self.block_buffer.push(self.current_line.clone());
+                        // Check if current_line is the closing fence (``` without trailing newline)
+                        let trimmed = self.current_line.trim_start();
+                        let leading_spaces = self.current_line.len() - trimmed.len();
+                        if trimmed == "```" && leading_spaces <= 3 {
+                            // This is the closing fence - don't include it in content
+                            // Just clear it and emit the block
+                        } else {
+                            self.block_buffer.push(self.current_line.clone());
+                        }
+                        self.current_line.clear();
                     }
                     self.emit_code_block();
                 }