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/Cargo.toml

@@ -27,6 +27,7 @@ chrono = { version = "0.4", features = ["serde"] }
 crossterm = "0.29.0"
 ratatui = "0.29"
 termimad = "0.34.0"
+regex = "1.10"
 
 [dev-dependencies]
 tempfile = "3.8"