Refine planner mode UI and error handling

Improve planner mode user experience with better error reporting,
cleaner tool output, and consistent log file placement.

- Propagate and display classified LLM errors to users with
  appropriate icons and context
- Display tool calls on single lines with truncated arguments
- Show LLM text responses without overwriting via UiWriter
- Ensure all logs write to workspace/logs directory consistently
- Set G3_WORKSPACE_PATH early in planning mode initialization

crates/g3-core/src/lib.rs

@@ -4011,7 +4011,7 @@ impl<W: UiWriter> Agent<W> {
                             // Skip printing tool call details for final_output
                             if tool_call.tool != "final_output" {
                                 // Tool call header
-                                self.ui_writer.print_tool_header(&tool_call.tool);
+                                self.ui_writer.print_tool_header(&tool_call.tool, Some(&tool_call.args));
                                 if let Some(args_obj) = tool_call.args.as_object() {
                                     for (key, value) in args_obj {
                                         let value_str = match value {

crates/g3-core/src/ui_writer.rs

@@ -21,7 +21,7 @@ pub trait UiWriter: Send + Sync {
     fn print_context_thinning(&self, message: &str);
 
     /// Print a tool execution header
-    fn print_tool_header(&self, tool_name: &str);
+    fn print_tool_header(&self, tool_name: &str, tool_args: Option<&serde_json::Value>);
 
     /// Print a tool argument
     fn print_tool_arg(&self, key: &str, value: &str);
@@ -81,7 +81,7 @@ impl UiWriter for NullUiWriter {
     fn print_system_prompt(&self, _prompt: &str) {}
     fn print_context_status(&self, _message: &str) {}
     fn print_context_thinning(&self, _message: &str) {}
-    fn print_tool_header(&self, _tool_name: &str) {}
+    fn print_tool_header(&self, _tool_name: &str, _tool_args: Option<&serde_json::Value>) {}
     fn print_tool_arg(&self, _key: &str, _value: &str) {}
     fn print_tool_output_header(&self) {}
     fn update_tool_output_line(&self, _line: &str) {}