feat: add external knowledge retrieval and cross-project activity

Connect Dify, FastGPT and RAGFlow through read-only bindings, encrypted local credentials, shared retrieval and persistent citations. Configuration and removal affect GoodBuddy only.

Show active conversations across projects, retain older live work, and improve project dialog scrolling. Queue concurrent questions and route managed SSH OpenCode questions through the Agent. Preserve recoverable read/search errors and partial search results.

Document the Electron UI automation standard, real model and external knowledge validation, and remaining acceptance limits.

Validation: earlier full regression passed 4081 tests with 66 skipped; subsequent focused remote-question checks and real Linux evidence are recorded in feature progress. Precommit typecheck and lint passed; latest project dialog and shared UI suites passed 58 tests. The full suite was not rerun for the final dialog changes.

Release note: Use external knowledge in conversations, find active work across projects, and answer concurrent local or managed SSH OpenCode questions. Managed SSH questions require the updated Agent adapter.

docs/features/knowledge-base/prd.md

@@ -13,6 +13,10 @@
 
 ## 1. 背景
 
+本文负责本地检索与分块增强。当前工作区另已接入外部实例、绑定和统一检索，其产品范围见
+[外部知识库 PRD](./external-knowledge-prd.md)，实现与待验收状态见[实施进度](./progress.md)。
+外部绑定不进入下述本地分块、向量索引和图谱维护流程。
+
 GoodBuddy 已具备本地多知识库、文件与目录同步、网页导入、SQLite FTS5、
 OpenAI 兼容向量模型、RRF 混合检索、知识图谱、任务状态和来源引用。现有实现
 优先建立了本地数据主权、安全边界和跨 Runtime 工具授权，但用户仍难以稳定