chore: flatten src/ to root

utils/messagePredicates.ts

@@ -0,0 +1,8 @@
+import type { Message, UserMessage } from '../types/message.js'
+
+// tool_result messages share type:'user' with human turns; the discriminant
+// is the optional toolUseResult field. Four PRs (#23977, #24016, #24022,
+// #24025) independently fixed miscounts from checking type==='user' alone.
+export function isHumanTurn(m: Message): m is UserMessage {
+  return m.type === 'user' && !m.isMeta && m.toolUseResult === undefined
+}