Remove legacy logs/ directory, consolidate all data under .g3/

This change removes the legacy logs/ directory and consolidates all
session data, error logs, and discovery files under the .g3/ directory.

New directory structure:
- .g3/sessions/<session_id>/session.json - session logs
- .g3/errors/ - error logs (was logs/errors/)
- .g3/background_processes/ - background process logs
- .g3/discovery/ - planner discovery files (was workspace/logs/)

Changes:
- paths.rs: Remove get_logs_dir()/logs_dir(), add get_errors_dir(),
  get_background_processes_dir(), get_discovery_dir()
- session.rs: Anonymous sessions now use .g3/sessions/anonymous_<ts>/
- error_handling.rs: Errors now saved to .g3/errors/
- project.rs: Remove logs_dir() and ensure_logs_dir() methods
- feedback_extraction.rs: Remove logs_dir field and fallback logic
- planner: Use .g3/ for workspace data and .g3/discovery/ for reports
- flock.rs: Look for session metrics in .g3/sessions/
- coach_feedback.rs: Remove fallback to logs/ path
- Update all tests to use new paths
- Update README.md and .gitignore

crates/g3-planner/tests/retry_feedback_test.rs

@@ -82,7 +82,6 @@ fn test_extracted_feedback_approval_detection() {
 fn test_feedback_extraction_config_default() {
     let config = FeedbackExtractionConfig::default();
     assert!(!config.verbose);
-    assert!(config.logs_dir.is_none());
     assert!(config.default_feedback.contains("review"));
 }
 
@@ -90,14 +89,9 @@ fn test_feedback_extraction_config_default() {
 fn test_feedback_extraction_config_custom() {
     let config = FeedbackExtractionConfig {
         verbose: true,
-        logs_dir: Some(std::path::PathBuf::from("/tmp/test_logs")),
         default_feedback: "Custom fallback message for testing".to_string(),
     };
     assert!(config.verbose);
-    assert_eq!(
-        config.logs_dir,
-        Some(std::path::PathBuf::from("/tmp/test_logs"))
-    );
     assert!(config.default_feedback.contains("Custom fallback"));
 }