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.

crates/g3-cli/src/lib.rs

@@ -405,6 +405,7 @@ pub async fn run() -> Result<()> {
         let codepath = cli.codepath.clone();
         return g3_planner::run_planning_mode(
             codepath,
+            cli.workspace.clone(),
             cli.no_git,
             cli.config.as_deref(),
         )