TurnResult: remove text field, simplify oneshot loop

- Remove TurnResult.text (was dead code - Agent::turn handles text internally)
- Simplify run_with_backend to just iterate over steps (Agent::turn loops
  for tool calls and handles empty responses internally)
- Change run/run_shared/run_forked_shared to return Result<(), String>
- Remove AgentResult.output field (no callers used it)
- Stub out legacy text-parsing code (audit, compare) that needs redesign
- Update digest.rs to not depend on text return
- Add level parameter to journal_new/journal_update for digest support

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/mind/subconscious.rs

@@ -319,7 +319,7 @@ struct SubconsciousAgent {
     forked_agent: Option<Arc<crate::agent::Agent>>,
     /// Entry index where the fork diverged from the conscious agent.
     fork_point: usize,
-    handle: Option<tokio::task::JoinHandle<(AutoAgent, Result<String, String>)>>,
+    handle: Option<tokio::task::JoinHandle<(AutoAgent, Result<(), String>)>>,
 }
 
 impl SubconsciousAgent {