refactor: improve readability of streaming parser and JSON filter

Agent: carmack

Changes:
- streaming_parser.rs: Unified find_first/last_tool_call_start into single
  find_tool_call_start with SearchDirection enum, reducing duplication.
  Simplified is_json_invalidated from 45 to 20 lines with clearer logic.
  Fixed redundant !escape_next check in find_complete_json_object_end.

- filter_json.rs: Simplified check_tool_pattern from 40 to 24 lines.
  Replaced repetitive prefix checks with loop over ["t", "to", "too", "tool"].
  Reduced trailing return statements with direct expression returns.

- ui_writer_impl.rs: Added ansi module for duration color constants.
  Simplified duration_color function by removing redundant comments.

- language_prompts.rs: Fixed test assertions to match actual prompt content
  ("obvious, readable Racket" instead of "RACKET-SPECIFIC GUIDANCE").

All 174+ tests pass. No behavior changes.

crates/g3-cli/src/language_prompts.rs

@@ -224,7 +224,7 @@ mod tests {
     fn test_carmack_racket_prompt_embedded() {
         let prompt = get_agent_language_prompt("carmack", "racket");
         assert!(prompt.is_some());
-        assert!(prompt.unwrap().contains("RACKET-SPECIFIC GUIDANCE"));
+        assert!(prompt.unwrap().contains("obvious, readable Racket"));
     }
 
     #[test]
@@ -242,6 +242,6 @@ mod tests {
         let prompts = get_agent_language_prompts_for_workspace(temp_dir.path(), "carmack");
         assert!(prompts.is_some());
         let content = prompts.unwrap();
-        assert!(content.contains("RACKET-SPECIFIC GUIDANCE"));
+        assert!(content.contains("obvious, readable Racket"));
     }
 }