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/tests/streaming_markdown_test.rs

@@ -1879,3 +1879,49 @@ fn strip_ansi(s: &str) -> String {
     let re = regex::Regex::new(r"\x1b\[[0-9;]*m").unwrap();
     re.replace_all(s, "").to_string()
 }
+
+#[test]
+fn test_code_fence_after_blank_line() {
+    let skin = MadSkin::default();
+    let mut fmt = StreamingMarkdownFormatter::new(skin);
+    
+    // Simulate the exact input from the bug - text followed by blank line followed by code fence
+    let input = "Done! The agent mode header now looks like:\n\n```\n>> agent mode | fowler\n```\n";
+    
+    // Process character by character like streaming would
+    let mut output = String::new();
+    for ch in input.chars() {
+        let chunk = fmt.process(&ch.to_string());
+        output.push_str(&chunk);
+    }
+    output.push_str(&fmt.finish());
+    
+    println!("Input: {:?}", input);
+    println!("Output: {:?}", output);
+    
+    // Check if backticks appear literally - they shouldn't
+    assert!(!output.contains("```"), "Literal backticks should not appear in output. Got: {}", output);
+}
+
+#[test]
+fn test_code_fence_no_trailing_newline() {
+    // Test code fence without trailing newline after closing ```
+    let skin = MadSkin::default();
+    let mut fmt = StreamingMarkdownFormatter::new(skin);
+    
+    // Note: no newline after closing ```
+    let input = "Done!\n\n```\n>> agent mode | fowler\n-> ~/src/g3\n   ✓ README | ✓ AGENTS.md | ✓ Memory\n```";
+    
+    let mut output = String::new();
+    for ch in input.chars() {
+        let chunk = fmt.process(&ch.to_string());
+        output.push_str(&chunk);
+    }
+    output.push_str(&fmt.finish());
+    
+    println!("Input: {:?}", input);
+    println!("Output: {:?}", output);
+    
+    // The closing ``` should NOT appear literally
+    assert!(!output.contains("```"), "Literal backticks in output: {}", output);
+}