scaffold: M0 skeleton + agent onboarding (build, architecture, progress)

Turn the design into a buildable, dependency-free M0 skeleton plus the
onboarding layer so a new agent can pick up instantly.

Build system:
- CMake + CMakePresets (dev = no deps; server-release = vcpkg) + vcpkg.json
- Skeleton builds with just a C++20 compiler; deps stay off until needed
- .gitattributes (LF), .gitignore, .clang-format

Core (libvoicecat):
- core/include/voicecat.h: full C ABI (the client/server contract), stubbed
- core/proto/voicecat.proto: control-plane wire format, matches docs/protocol.md
- src/{net,crypto,codec,protocol,session,audio,core}: subsystem stubs that
  return VC_ERR_NOT_IMPLEMENTED, each pointing to its design doc
- server/ (voicecat-server) and tools/vccli/ link the core
- tests/: CTest smoke test asserting the C ABI contract (behavior, not just build)
- clients/{apple,windows}: M4 placeholders

Onboarding for agents:
- CLAUDE.md: hub — build/test commands, architecture at a glance, doc map, rules
- AGENTS.md: working method (behavior-driven; clean compile is the floor not the goal)
- PROGRESS.md: living tracker — M0 done, M1 task checklist, "where we left off"

Verified: cmake --preset dev && cmake --build --preset dev && ctest --preset dev → green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tools/vccli/src/main.cpp

@@ -0,0 +1,47 @@
+/*
+ * vccli — headless test client.
+ *
+ * This is the primary way the protocol is exercised and verified from M1 onward (see
+ * AGENTS.md). Each milestone's exit criterion is demonstrated by driving two vccli
+ * instances against a real voicecat-server. Today it just shows the C ABI is linkable.
+ */
+#include <cstdio>
+
+#include "voicecat.h"
+
+namespace {
+
+void on_event(void* /*user*/, const vc_event* ev) {
+    std::printf("[event] type=%d state=%d result=%d text=%s\n", ev->type, ev->connection_state,
+                ev->result, ev->text ? ev->text : "");
+}
+
+}  // namespace
+
+int main(int argc, char** argv) {
+    std::printf("vccli — VoiceCat test client (core %s, protocol v%d)\n", vc_version_string(),
+                VOICECAT_PROTOCOL_VERSION);
+
+    vc_config cfg{};
+    cfg.client_name = "vccli";
+    cfg.client_version = vc_version_string();
+    cfg.log_level = VC_LOG_INFO;
+
+    vc_callbacks cb{};
+    cb.on_event = on_event;
+
+    vc_client* c = vc_client_create(&cfg, cb);
+    if (c == nullptr) {
+        std::fprintf(stderr, "failed to create client\n");
+        return 1;
+    }
+
+    // M0: demonstrate the call surface. These return VC_ERR_NOT_IMPLEMENTED for now.
+    const char* host = (argc > 1) ? argv[1] : "127.0.0.1";
+    vc_result r = vc_connect(c, host, 8384);
+    std::printf("vc_connect(%s:8384) -> %d (%s)\n", host, r, vc_result_string(r));
+
+    vc_client_destroy(c);
+    std::printf("ok\n");
+    return 0;
+}