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-cli/src/coach_feedback.rs

@@ -54,12 +54,7 @@ pub fn extract_from_logs(
 
 /// Resolve the log file path, trying new path first then falling back to old.
 fn resolve_log_path(session_id: &str) -> std::path::PathBuf {
-    let new_path = g3_core::get_session_file(session_id);
-    if new_path.exists() {
-        new_path
-    } else {
-        Path::new("logs").join(format!("g3_session_{}.json", session_id))
-    }
+    g3_core::get_session_file(session_id)
 }
 
 /// Extract feedback from a session log file.