Refactor to plugin architecture with B3 CLI UX, C2 smoke tests, C3 CI scripts

Architecture overhaul (Wave 1-4 collaborative work):
- Migrated dstalk-core from monolithic api.cpp to plugin-based design with
  host/service_registry/event_bus/plugin_loader and topological initialization.
- Split public headers into dstalk_host.h / dstalk_services.h /
  dstalk_lsp.h / dstalk_types.h; deleted obsolete dstalk_api.h and inlined
  TLS/file/net code now provided by plugins.
- Added 9 plugins: deepseek, anthropic, network, session, context, tools,
  config, file-io, lsp; AI plugins register as "ai.<provider>" services.

B3 CLI interaction enhancement:
- Prompt now shows current model name (A1).
- /status command prints model/base_url/api_key (sanitized: shown only
  as set/unset)/services readiness (A2).
- SIGINT/Ctrl+C handled on POSIX (signal) and Windows (SetConsoleCtrlHandler);
  /quit no longer std::exit(0) but sets a quit flag so dstalk_shutdown runs
  exactly once via natural control flow (B1+B2).
- Cross-DLL free fixed: print_file uses dstalk_free instead of std::free (B4).
- --batch mode plus isatty auto-detection for piped stdin (C1).
- fgets truncation detection with friendly error and stdin draining (C3).
- Distinct exit codes (init/AI/service-unavailable) (C4).
- /model rejects empty model name (C5).

C2 smoke test extension:
- 4 new test blocks: null-safety (file_io/session/tools/config),
  escape-boundary round-trip, tools->execute call chain, session robustness
  (add(nullptr), clear -> token_count == 0).

C3 CI build scripts:
- scripts/ci-build.sh and scripts/ci-build.bat invoke cmake configure +
  parallel build + ctest, suitable for GitHub Actions.

Build verified: dstalk-cli compiles, smoke test passes via ctest.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

plugins/CMakeLists.txt

@@ -0,0 +1,18 @@
+# ============================================================
+# 插件目录 — 所有功能插件
+# ============================================================
+
+# 基础插件（无外部服务依赖）
+add_subdirectory(config)
+add_subdirectory(file-io)
+add_subdirectory(network)
+
+# 中间插件（依赖基础插件）
+add_subdirectory(session)
+add_subdirectory(context)
+
+# 上层插件（依赖中间插件）
+add_subdirectory(deepseek)
+add_subdirectory(anthropic)
+add_subdirectory(tools)
+add_subdirectory(lsp)