Fix four review findings: busy release, poison recovery, core.md errors, chat loop

Implemented-By: OpenCode session (model recorded in docs/implementer-log.md)

crates/proto/src/ids.rs

@@ -42,6 +42,13 @@ impl SessionId {
     }
 }
 
+impl Default for SessionId {
+    fn default() -> Self {
+        // Not the empty string, which `new` rejects: a fallback must still be a valid id.
+        SessionId("chat-0-0".to_string())
+    }
+}
+
 impl TryFrom<String> for SessionId {
     type Error = ValueError;