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/file-io/src/file_io_plugin.cpp

@@ -0,0 +1,95 @@
+#include "dstalk/dstalk_host.h"
+#include "dstalk/dstalk_services.h"
+
+#include <cstdio>
+#include <cstdlib>
+#include <cstring>
+
+// ============================================================
+// Global state
+// ============================================================
+static const dstalk_host_api_t* g_host = nullptr;
+
+// ============================================================
+// Service implementations
+// ============================================================
+static int file_read(const char* path, char** content) {
+    if (!path || !content) return -1;
+
+    FILE* fp = fopen(path, "rb");
+    if (!fp) return -1;
+
+    // Get file size
+    fseek(fp, 0, SEEK_END);
+    long fsize = ftell(fp);
+    fseek(fp, 0, SEEK_SET);
+
+    if (fsize < 0) {
+        fclose(fp);
+        return -1;
+    }
+
+    // Allocate buffer (+1 for null terminator)
+    char* buf = (char*)malloc((size_t)fsize + 1);
+    if (!buf) {
+        fclose(fp);
+        return -1;
+    }
+
+    size_t read_bytes = fread(buf, 1, (size_t)fsize, fp);
+    fclose(fp);
+
+    if (read_bytes != (size_t)fsize) {
+        free(buf);
+        return -1;
+    }
+
+    buf[read_bytes] = '\0';
+    *content = buf;
+    return 0;
+}
+
+static int file_write(const char* path, const char* content) {
+    if (!path || !content) return -1;
+
+    FILE* fp = fopen(path, "wb");
+    if (!fp) return -1;
+
+    size_t len = strlen(content);
+    size_t written = fwrite(content, 1, len, fp);
+    fclose(fp);
+
+    return (written == len) ? 0 : -1;
+}
+
+static dstalk_file_io_service_t g_service = {
+    file_read,
+    file_write
+};
+
+// ============================================================
+// Plugin lifecycle
+// ============================================================
+static int on_init(const dstalk_host_api_t* host) {
+    g_host = host;
+    return host->register_service("file_io", 1, &g_service);
+}
+
+static void on_shutdown() {
+    // nothing to clean up
+}
+
+static dstalk_plugin_info_t g_info = {
+    "file-io",             // name
+    "1.0.0",               // version
+    "Basic file I/O service",  // description
+    DSTALK_API_VERSION,    // api_version
+    {nullptr},             // dependencies (none)
+    on_init,               // on_init
+    on_shutdown,           // on_shutdown
+    nullptr                // on_event
+};
+
+extern "C" DSTALK_PLUGIN_EXPORT dstalk_plugin_info_t* dstalk_plugin_init(void) {
+    return &g_info;
+}