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>

.gitignore

@@ -0,0 +1,40 @@
+# Build output
+/build/
+/out/
+*.o
+*.obj
+*.a
+*.lib
+*.so
+*.dylib
+*.dll
+*.exe
+*.pdb
+
+# vcpkg
+/vcpkg_installed/
+/vcpkg/
+
+# Generated protobuf
+*.pb.cc
+*.pb.h
+*_pb2.py
+
+# Server runtime data (self-host data dir, keys, db)
+/voicecat-data/
+*.sqlite
+*.sqlite-*
+
+# IDE / OS
+.vs/
+.vscode/
+.idea/
+*.user
+.DS_Store
+Thumbs.db
+
+# Apple / Windows client build artifacts (added in M4)
+clients/apple/**/build/
+clients/apple/**/*.xcodeproj/xcuserdata/
+clients/windows/**/bin/
+clients/windows/**/obj/