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>

core/CMakeLists.txt

@@ -0,0 +1,43 @@
+# libvoicecat — the shared C++ core (docs/architecture.md).
+# Sources are globbed so adding a stub under src/<subsystem>/ needs no CMake edit.
+file(GLOB_RECURSE VOICECAT_SOURCES CONFIGURE_DEPENDS
+     "${CMAKE_CURRENT_SOURCE_DIR}/src/*.cpp")
+
+if(VOICECAT_BUILD_SHARED)
+    add_library(voicecat SHARED ${VOICECAT_SOURCES})
+else()
+    add_library(voicecat STATIC ${VOICECAT_SOURCES})
+    # Static consumers must see VC_API as empty (no dllimport).
+    target_compile_definitions(voicecat PUBLIC VOICECAT_STATIC)
+endif()
+add_library(voicecat::voicecat ALIAS voicecat)
+
+target_include_directories(voicecat
+    PUBLIC  ${CMAKE_CURRENT_SOURCE_DIR}/include
+    PRIVATE ${CMAKE_CURRENT_SOURCE_DIR}/src)
+
+target_compile_definitions(voicecat PRIVATE VOICECAT_BUILDING)
+target_compile_features(voicecat PUBLIC cxx_std_20)
+
+set_target_properties(voicecat PROPERTIES
+    C_VISIBILITY_PRESET hidden
+    CXX_VISIBILITY_PRESET hidden
+    VISIBILITY_INLINES_HIDDEN ON)
+
+if(VOICECAT_USE_VCPKG_DEPS)
+    # Wire real dependencies here as each subsystem is implemented. Example (uncomment
+    # the ones a subsystem needs; see docs/tech-stack.md and core/proto for protobuf):
+    #   find_package(unofficial-sodium CONFIG REQUIRED)   # crypto/  (libsodium)
+    #   find_package(MbedTLS CONFIG REQUIRED)             # crypto/  (TLS 1.3)
+    #   find_package(Opus CONFIG REQUIRED)                # codec/
+    #   find_package(protobuf CONFIG REQUIRED)            # protocol/
+    #   find_package(asio CONFIG REQUIRED)                # net/
+    #   find_package(unofficial-sqlite3 CONFIG REQUIRED)  # server persistence
+    #   find_package(spdlog CONFIG REQUIRED)
+    #   target_link_libraries(voicecat PRIVATE Opus::opus protobuf::libprotobuf ...)
+    #
+    # protobuf codegen (when protocol/ is implemented):
+    #   find_package(Protobuf CONFIG REQUIRED)
+    #   protobuf_generate(TARGET voicecat PROTOS proto/voicecat.proto LANGUAGE cpp)
+    message(STATUS "voicecat: deps ON — add find_package()/link calls here as subsystems land")
+endif()