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/src/codec/opus_codec.h

@@ -0,0 +1,39 @@
+/*
+ * codec/opus_codec.h — Opus encode/decode (libopus 1.6).
+ *
+ * Design: docs/voice.md §3–4. Per-channel AudioConfig (mono/stereo, bitrate, frame size,
+ * FEC, DTX, complexity). The server relays Opus payloads unmodified (no transcode).
+ *
+ * STATUS: M0 stub.
+ */
+#ifndef VOICECAT_CODEC_OPUS_CODEC_H
+#define VOICECAT_CODEC_OPUS_CODEC_H
+
+#include <cstdint>
+
+namespace voicecat::codec {
+
+struct OpusParams {
+    uint32_t sample_rate = 48000;
+    uint32_t bitrate_bps = 24000;
+    uint32_t frame_ms = 20;
+    bool stereo = false;
+    bool fec = true;
+    bool dtx = true;
+    uint32_t complexity = 10;
+    uint32_t expected_packet_loss = 0;
+};
+
+class OpusEncoder {
+   public:
+    // TODO(M2): init(params); encode(pcm, frame) -> opus bytes.
+};
+
+class OpusDecoder {
+   public:
+    // TODO(M2): init(params); decode(opus, out_pcm); PLC on loss; FEC from next packet.
+};
+
+}  // namespace voicecat::codec
+
+#endif  // VOICECAT_CODEC_OPUS_CODEC_H