Unify final_output rendering with streaming markdown formatter

Replace the separate syntax_highlight module with the streaming markdown
formatter for final_output rendering. This:

- Removes special buffered rendering logic for final_output
- Uses the same StreamingMarkdownFormatter used for agent responses
- Removes the spinner animation (content renders immediately)
- Deletes the now-unused syntax_highlight.rs module
- Updates test to use the streaming formatter

Benefits:
- Consistent rendering across all markdown output
- Less code to maintain (removed ~250 lines)
- Same syntax highlighting via syntect (already in streaming formatter)

crates/g3-cli/src/lib.rs

@@ -1,6 +1,5 @@
 // JSON tool call filtering for display (moved from g3-core)
 pub mod filter_json;
-pub mod syntax_highlight;
 pub mod streaming_markdown;
 
 use anyhow::Result;