Add template variable injection for --include-prompt

Supports {{var}} syntax for variable substitution in included prompt files.

Currently supported variables:
- {{today}}: Current date in ISO format (YYYY-MM-DD)

Unknown variables trigger a warning and are left unchanged.

- Add template.rs module with process_template() function
- Integrate template processing into read_include_prompt()
- Add comprehensive tests for template processing

crates/g3-cli/src/lib.rs

@@ -20,6 +20,7 @@ mod task_execution;
 mod ui_writer_impl;
 mod utils;
 mod g3_status;
+mod template;
 mod completion;
 
 use anyhow::Result;