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/agent/mod.rs

@@ -23,7 +23,7 @@ use std::sync::Arc;
 use anyhow::Result;
 
 use api::ApiClient;
-use context::{AstNode, NodeBody, ContextState, Section, Ast, PendingToolCall, ResponseParser, Role};
+use context::{AstNode, ContextState, Section, Ast, PendingToolCall, ResponseParser, Role};
 
 use crate::mind::log::ConversationLog;
 
@@ -105,9 +105,6 @@ pub async fn start_activity(agent: &Arc<Agent>, label: impl Into<String>) -> Act
 
 /// Result of a single agent turn.
 pub struct TurnResult {
-    /// The text response (already sent through UI channel).
-    #[allow(dead_code)]
-    pub text: String,
     /// Whether the model called yield_to_user during this turn.
     pub yield_requested: bool,
     /// Whether any tools (other than yield_to_user) were called.
@@ -445,24 +442,12 @@ impl Agent {
             }
 
             // Text-only response — extract text and return
-            let text = {
-                let ctx = agent.context.lock().await;
-                let children = ctx.conversation()[branch_idx].children();
-                children.iter()
-                    .filter_map(|c| c.leaf())
-                    .filter(|l| matches!(l.body(), NodeBody::Content(_)))
-                    .map(|l| l.body().text())
-                    .collect::<Vec<_>>()
-                    .join("")
-            };
-
             let mut st = agent.state.lock().await;
             if st.pending_yield { ds.yield_requested = true; st.pending_yield = false; }
             if st.pending_model_switch.is_some() { ds.model_switch = st.pending_model_switch.take(); }
             if st.pending_dmn_pause { ds.dmn_pause = true; st.pending_dmn_pause = false; }
 
             return Ok(TurnResult {
-                text,
                 yield_requested: ds.yield_requested,
                 had_tool_calls: ds.had_tool_calls,
                 tool_errors: ds.tool_errors,