Add tokens consumed and context percentage to per-tool timing footer

The per-tool timing line now shows:
- Tokens delta (tokens added to context by this tool call)
- Context window usage percentage

Example: └─ ⚡️ 1ms  523tk | 49% ctx

Changes:
- Updated UiWriter trait print_tool_timing signature
- Track tokens before/after adding tool messages to calculate delta
- Updated ConsoleUiWriter, MachineUiWriter, PlannerUiWriter, and test mocks

crates/g3-planner/src/llm.rs

@@ -265,7 +265,7 @@ impl g3_core::ui_writer::UiWriter for PlannerUiWriter {
     fn update_tool_output_line(&self, _line: &str) {}
     fn print_tool_output_line(&self, _line: &str) {}
     fn print_tool_output_summary(&self, _hidden_count: usize) {}
-    fn print_tool_timing(&self, _duration_str: &str) {}
+    fn print_tool_timing(&self, _duration_str: &str, _tokens_delta: u32, _context_percentage: f32) {}
     
     fn print_agent_prompt(&self) {
         // No-op - don't add extra blank lines