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/digest.rs

@@ -286,7 +286,7 @@ fn generate_digest(
     };
     let digest = crate::agent::oneshot::call_api_with_tools_sync(
         &def.agent, &prompts, &phases, def.temperature, def.priority,
-        &tools, None, &log)?;
+        &tools, None)?;
 
     let key = digest_node_key(level.name, label);
     store.upsert_provenance(&key, &digest, "digest:write")?;