refactor(cli): extract display utilities to eliminate code duplication

Created display.rs module with shared display functions:
- format_workspace_path() / print_workspace_path()
- LoadedContent struct for tracking loaded project files
- print_loaded_status() for status line display
- print_project_heading() for README heading

Updated interactive.rs and agent_mode.rs to use the new module,
eliminating duplicated workspace path formatting and loaded items
status line logic.

Results:
- interactive.rs: 641 → 595 lines (-46)
- agent_mode.rs: 312 → 288 lines (-24)
- New display.rs: 197 lines with 5 unit tests

Agent: fowler

crates/g3-cli/src/lib.rs

@@ -12,6 +12,7 @@ mod agent_mode;
 mod autonomous;
 mod cli_args;
 mod coach_feedback;
+mod display;
 mod interactive;
 mod simple_output;
 mod task_execution;