feat: nation-op rename review — confirm/reject routes, client dialog, AGENTS.md (P4a, #16)

Wire the 'LLM proposes, player confirms' nation-rename flow end to end:
- server: POST /api/games/:id/nation-op/confirm|reject (404/400, version-bumped SSE state)
- client: confirmNationOp/rejectNationOp api helpers + NationOpDialog (editable rename) wired into the Action panel via a 'Nation proposals' trigger
- AGENTS.md: document the pendingNationOps mechanism, routes, nationRef resolution, stub-never-proposes

server/engine.test.ts

@@ -4,7 +4,6 @@ import { makeEvent, type NationOp, type TurnResult } from '../shared/protocol';
 import type { LlmClient, ToolDefinition, WireMessage } from './llm';
 import { makeStubLlmClient } from './stub';
 import {
-  applyNationOp,
   applyTurnResult,
   buildSystemPrompt,
   buildUserPrompt,