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/src/project.rs

@@ -127,18 +127,4 @@ impl Project {
         std::env::set_current_dir(&self.workspace_dir)?;
         Ok(())
     }
-
-    /// Get the logs directory for the project
-    pub fn logs_dir(&self) -> PathBuf {
-        self.workspace_dir.join("logs")
-    }
-
-    /// Ensure the logs directory exists
-    pub fn ensure_logs_dir(&self) -> Result<()> {
-        let logs_dir = self.logs_dir();
-        if !logs_dir.exists() {
-            std::fs::create_dir_all(&logs_dir)?;
-        }
-        Ok(())
-    }
 }