feat: Add LSP plugin unit tests and frontend common initialization library

- Introduced `dstalk_lsp_plugin_test` for testing LSP plugin functionalities including `lsp_trim`, `lsp_frame_message`, and `lsp_parse_content_length`.
- Created `dstalk_frontend_common` static library to encapsulate shared initialization logic for frontend components (CLI, GUI, Web).
- Implemented configuration file discovery and service querying in `dstalk_frontend_init`.
- Added internal headers for LSP and Anthropic plugins to facilitate unit testing.
- Established a mailroom system for asynchronous message passing between stateless agents, enhancing coordination and context management.

dstalk_cli/CMakeLists.txt

@@ -15,3 +15,9 @@ find_package(Boost REQUIRED CONFIG)
 target_link_libraries(dstalk_cli
     PRIVATE dstalk boost::boost dstalk_boost_config
 )
+
+# POSIX 平台需要 pthread (用于 std::thread spinner)
+if(NOT WIN32)
+    find_package(Threads REQUIRED)
+    target_link_libraries(dstalk_cli PRIVATE Threads::Threads)
+endif()