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

crates/g3-core/src/ui_writer.rs

@@ -69,6 +69,18 @@ pub trait UiWriter: Send + Sync {
     /// Print the final output summary with markdown formatting
     /// Shows a spinner while formatting, then renders the markdown
     fn print_final_output(&self, summary: &str);
+
+    /// Filter JSON tool calls from streaming content for display.
+    /// This is a UI concern - the raw content should be preserved for logging.
+    /// Default implementation passes through unchanged.
+    fn filter_json_tool_calls(&self, content: &str) -> String {
+        content.to_string()
+    }
+
+    /// Reset the JSON tool call filter state.
+    /// Called at the start of a new response to clear any partial state.
+    /// Default implementation does nothing.
+    fn reset_json_filter(&self) {}
 }
 
 /// A no-op implementation for when UI output is not needed