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-core/tests/integration_blackbox_test.rs

@@ -53,7 +53,7 @@ done
             fs::set_permissions(&script_path, fs::Permissions::from_mode(0o755)).unwrap();
         }
 
-        let log_dir = test_dir.join("logs");
+        let log_dir = test_dir.join(".g3").join("background_processes");
         let manager = BackgroundProcessManager::new(log_dir);
 
         // Start the process
@@ -116,7 +116,7 @@ sleep 30
             fs::set_permissions(&script_path, fs::Permissions::from_mode(0o755)).unwrap();
         }
 
-        let log_dir = test_dir.join("logs");
+        let log_dir = test_dir.join(".g3").join("background_processes");
         let manager = BackgroundProcessManager::new(log_dir);
 
         // Start a process
@@ -151,7 +151,7 @@ sleep 30
         let _ = fs::remove_dir_all(&test_dir);
         fs::create_dir_all(&test_dir).unwrap();
 
-        let log_dir = test_dir.join("logs");
+        let log_dir = test_dir.join(".g3").join("background_processes");
         let manager = BackgroundProcessManager::new(log_dir);
 
         // Getting a process that doesn't exist should return None