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-cli/src/ui_writer_impl.rs

@@ -1,3 +1,4 @@
+use crate::fixed_filter_json::{fixed_filter_json_tool_calls, reset_fixed_json_tool_state};
 use g3_core::ui_writer::UiWriter;
 use std::io::{self, Write};
 use termimad::MadSkin;
@@ -350,4 +351,14 @@ impl UiWriter for ConsoleUiWriter {
         println!();
         println!("\x1b[1;35m━━━━━━━━━━━━━━━\x1b[0m");
     }
+
+    fn filter_json_tool_calls(&self, content: &str) -> String {
+        // Apply JSON tool call filtering for display
+        fixed_filter_json_tool_calls(content)
+    }
+
+    fn reset_json_filter(&self) {
+        // Reset the filter state for a new response
+        reset_fixed_json_tool_state();
+    }
 }