Kill log callback — use ConversationEntry::Log for debug traces

Add Log variant to ConversationEntry that serializes to the
conversation log but is filtered out on read-back and API calls.
AutoAgent writes debug/status info (turns, tokens, tool calls)
through the conversation log instead of a callback parameter.

Removes the log callback from run_one_agent, call_api_with_tools,
and all callers.

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

src/subconscious/consolidate.rs

@@ -74,7 +74,7 @@ pub fn consolidate_full_with_progress(
             *store = Store::load()?;
         }
 
-        match oneshot::run_one_agent(store, agent_type, *count, None, &|_| {}) {
+        match oneshot::run_one_agent(store, agent_type, *count, None) {
             Ok(_) => {
                 let msg = "  Done".to_string();
                 log_line(&mut log_buf, &msg);