Document retry config location and verify planning mode logic

Add documentation for retry configuration in planning mode:
- Document retry settings in .g3.toml under [agent] section
- Note RetryConfig implementation in g3-core/src/retry.rs
- Clarify hardcoded vs config-based retry values

Verify existing retry loop and coach feedback parsing:
- Confirm execute_with_retry() handles recoverable errors
- Document feedback extraction source priority order
- Provide manual verification steps for testing

crates/g3-core/src/lib.rs

@@ -1,10 +1,14 @@
 pub mod code_search;
 pub mod error_handling;
+pub mod feedback_extraction;
 pub mod project;
+pub mod retry;
 pub mod task_result;
 pub mod ui_writer;
 
 pub use task_result::TaskResult;
+pub use retry::{RetryConfig, RetryResult, execute_with_retry, retry_operation};
+pub use feedback_extraction::{ExtractedFeedback, FeedbackSource, FeedbackExtractionConfig, extract_coach_feedback};
 
 #[cfg(test)]
 mod task_result_comprehensive_tests;