Fix planner UI whitespace and workspace logs directory

Resolve two critical issues in planner mode that persisted through
multiple fix attempts:

1. Remove excessive whitespace between tool call displays by replacing
   direct println!() calls with ui_writer methods and eliminating
   redundant newlines in agent response streaming.

2. Ensure all log files (errors, sessions, tool calls, context dumps)
   are written to <workspace>/logs instead of codepath by properly
   initializing G3_WORKSPACE_PATH from --workspace argument.

g3-plan/new_requirements.md

@@ -0,0 +1,7 @@
+# Test Requirements
+
+Create a simple test to verify that:
+1. Tool calls display without extra blank lines
+2. Log files are written to the workspace directory
+
+Just list the files in the current directory and read one file.