Move fixed_filter_json from g3-core to g3-cli

Properly separates UI display concern from core library:
- fixed_filter_json module now lives in g3-cli (UI layer)
- UiWriter trait gains filter_json_tool_calls() and reset_json_filter() methods
- g3-core delegates filtering to UI layer via trait methods
- Different UiWriter implementations can choose their own filtering behavior
- ConsoleUiWriter filters JSON tool calls for clean terminal display
- MachineUiWriter/NullUiWriter use default pass-through

Benefits:
- Proper separation of concerns
- Core stays clean without display-specific logic
- Testability - filter can be tested independently in g3-cli

Cargo.lock

@@ -1373,6 +1373,7 @@ dependencies = [
  "hex",
  "indicatif",
  "ratatui",
+ "regex",
  "rustyline",
  "serde",
  "serde_json",