feat(agent): 检索工具、模型接入与结构化回复

Agent 按组织与知识源范围调用检索工具，密钥加密保存，回复含来源与转人工意图。

backend/app/agents/choice_prompt.py

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+"""
+Copyright 2024-2026 ReplyPier
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+
+When the agent may offer tappable choices (ChatResponse.quick_replies).
+
+Only given to agents on a channel that can show buttons, so every other
+surface keeps an unchanged prompt. It is appended to the agent's
+instructions — rules placed in a top-of-prompt policy block get ignored.
+"""
+
+from app.channels.constants import MAX_CHOICE_OPTIONS
+from app.channels.interactive_limits import InteractiveLimits
+
+
+def choice_instructions(limits: InteractiveLimits) -> str:
+    """What the agent is told about offering choices on a channel with these limits."""
+    most = min(limits.max_buttons, MAX_CHOICE_OPTIONS) if not limits.supports_list else MAX_CHOICE_OPTIONS
+    shown = (f"up to {limits.max_buttons} short options show as buttons, more as a list"
+             if limits.supports_list else "they show as buttons")
+    return (
+        "This customer's app can show tappable choices. When your reply asks them to "
+        "pick from a small, fixed set of options — a yes/no confirmation, which product, "
+        "a time slot — still ask the question in `message`, and put the options in "
+        f"`quick_replies`: at most {most}, each under {limits.button_title_max} characters "
+        f"where you can ({shown}). "
+        "Don't list the options again inside `message`. Leave `quick_replies` empty for "
+        "open questions, for answers they must type (a name, an email, an order number), "
+        "and when you are simply answering."
+    )