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/session/session.cpp

@@ -0,0 +1,8 @@
+#include "session/session.h"
+
+namespace voicecat::session {
+
+// M0 stub. Channel tree, users, streams, permissions, and ephemeral text relay land in M1.
+// See docs/protocol.md §5.
+
+}  // namespace voicecat::session

core/src/session/session.h

@@ -0,0 +1,55 @@
+/*
+ * session/session.h — domain model: channels, users, streams, permissions, text.
+ *
+ * Design: docs/protocol.md §5, docs/architecture.md §5. Shared by client (local mirror of
+ * server state) and server (authoritative). Text is ephemeral (no history). Accounts are
+ * admin-provisioned.
+ *
+ * STATUS: M0 stub.
+ */
+#ifndef VOICECAT_SESSION_SESSION_H
+#define VOICECAT_SESSION_SESSION_H
+
+#include <cstdint>
+#include <string>
+#include <vector>
+
+namespace voicecat::session {
+
+struct Channel {
+    uint32_t id = 0;
+    uint32_t parent_id = 0;
+    std::string name;
+    bool password_protected = false;
+    uint32_t max_users = 0;
+};
+
+struct Stream {
+    uint32_t stream_id = 0;
+    uint32_t ssrc = 0;
+    int kind = 0;  // vc_stream_kind
+    std::string label;
+};
+
+struct User {
+    uint32_t id = 0;
+    std::string nickname;
+    bool is_guest = true;
+    uint32_t channel_id = 0;
+    std::vector<Stream> streams;
+};
+
+// Mirror/authority for the channel tree + user list. TODO(M1): snapshot + delta apply.
+class SessionModel {
+   public:
+    const std::vector<Channel>& channels() const { return channels_; }
+    const std::vector<User>& users() const { return users_; }
+
+   private:
+    std::vector<Channel> channels_;
+    std::vector<User> users_;
+};
+
+}  // namespace voicecat::session
+
+#endif  // VOICECAT_SESSION_SESSION_H