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>

CMakeLists.txt

@@ -0,0 +1,50 @@
+cmake_minimum_required(VERSION 3.25)
+
+project(voicecat
+    VERSION 0.0.1
+    DESCRIPTION "Self-hosted native voice & text chat (see docs/)"
+    LANGUAGES CXX)
+
+# ── Options ───────────────────────────────────────────────────────────────────
+# The M0 skeleton compiles with NO third-party dependencies: every subsystem is a
+# stub that returns VC_ERR_NOT_IMPLEMENTED. As each subsystem is built out, flip
+# VOICECAT_USE_VCPKG_DEPS=ON so CMake pulls the real libraries (mbedTLS, libsodium,
+# opus, protobuf, ...) via the vcpkg toolchain (see vcpkg.json / docs/tech-stack.md).
+option(VOICECAT_USE_VCPKG_DEPS "Link real third-party deps via vcpkg" OFF)
+option(VOICECAT_BUILD_SERVER   "Build voicecat-server"                ON)
+option(VOICECAT_BUILD_TOOLS    "Build the vccli headless test client" ON)
+option(VOICECAT_BUILD_TESTS    "Build tests"                          ON)
+option(VOICECAT_BUILD_SHARED   "Build libvoicecat as a shared library" OFF)
+
+# ── Global settings ───────────────────────────────────────────────────────────
+set(CMAKE_CXX_STANDARD 20)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
+set(CMAKE_CXX_EXTENSIONS OFF)
+
+if(NOT CMAKE_BUILD_TYPE AND NOT CMAKE_CONFIGURATION_TYPES)
+    set(CMAKE_BUILD_TYPE Debug CACHE STRING "" FORCE)
+endif()
+
+set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/bin)
+set(CMAKE_LIBRARY_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/lib)
+set(CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/lib)
+
+# ── Targets ───────────────────────────────────────────────────────────────────
+add_subdirectory(core)
+
+if(VOICECAT_BUILD_SERVER)
+    add_subdirectory(server)
+endif()
+
+if(VOICECAT_BUILD_TOOLS)
+    add_subdirectory(tools/vccli)
+endif()
+
+if(VOICECAT_BUILD_TESTS)
+    enable_testing()
+    add_subdirectory(tests)
+endif()
+
+message(STATUS "VoiceCat ${PROJECT_VERSION} configured "
+        "(vcpkg deps: ${VOICECAT_USE_VCPKG_DEPS}, "
+        "server: ${VOICECAT_BUILD_SERVER}, tools: ${VOICECAT_BUILD_TOOLS})")