feat(ai): 步数触顶改为 incomplete 可续跑

MAX_AGENT_STEPS 触顶保留已完成 steps，支持追问续跑；
补充 docs 与 zh/en 文案 maxStepsIncomplete。

src/i18n/locales/en/errors.json

@@ -165,7 +165,8 @@
     "criticalBlocked": "Command hit a destructive hard block; no override: {{command}}",
     "execFailed": "Remote execution failed: {{error}}",
     "maxSteps": "Reached {{max}}-step limit; narrow the scope and retry",
-    "sshRequired": "A connected remote SSH session is required"
+    "sshRequired": "A connected remote SSH session is required",
+    "maxStepsIncomplete": "Incomplete (max_steps): hit the {{max}}-step budget. Narrow scope or follow up to resume."
   },
   "zmodem": {
     "readFailed": "Read failed: {{name}}",