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>

scripts/ci-build.sh

@@ -0,0 +1,29 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+PROJECT_DIR="$(cd "$(dirname "$0")/.." && pwd)"
+BUILD_DIR="${PROJECT_DIR}/build"
+
+echo "=== dstalk CI Build ==="
+echo "Project: ${PROJECT_DIR}"
+
+# 创建构建目录
+mkdir -p "${BUILD_DIR}"
+cd "${BUILD_DIR}"
+
+# CMake 配置
+echo "--- CMake Configure ---"
+cmake "${PROJECT_DIR}" -G Ninja \
+    -DCMAKE_BUILD_TYPE=Release \
+    -DDSTALK_BUILD_TESTS=ON \
+    -DDSTALK_BUILD_GUI=OFF
+
+# 编译
+echo "--- Build ---"
+cmake --build . --parallel
+
+# 运行测试
+echo "--- Test ---"
+ctest --output-on-failure --parallel 4
+
+echo "=== CI Build PASSED ==="