feat(deploy): Docker + Linux server deployment

Multi-stage Dockerfile (builder → export → runtime) producing a 149 MB
Ubuntu 24.04 image, verified booting end-to-end on Docker Desktop.  vcpkg
fetched via shallow git fetch at the pinned baseline, release-only overlay
triplets (x64-linux, arm64-linux) to halve intermediate disk usage, and
buildtrees deleted within the RUN layer so they never land in the image or
the BuildKit cache.  Binary cache mount (VCPKG_BINARY_SOURCES) makes
subsequent rebuilds restore pre-built packages instead of recompiling.

Also adds:
- docker-compose.yml for one-command local deploy
- .dockerignore (excludes clients/, build/, .git/)
- .github/workflows/build-linux.yml — CI cross-build for amd64 + arm64
  with downloadable artifacts (primary path for building from Windows)
- scripts/build-linux-binaries.sh — local Docker binary extraction fallback
- deploy/linux/voicecat.service — hardened systemd unit for bare-metal
- cmake/voicecat-toolchain.cmake now auto-wires VCPKG_OVERLAY_TRIPLETS

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -0,0 +1,16 @@
+services:
+  voicecat:
+    build: .
+    image: ghcr.io/org/voicecat:latest
+    container_name: voicecat
+    restart: unless-stopped
+    ports:
+      - "8384:8384/tcp"
+      - "8384:8384/udp"
+    volumes:
+      - voicecat-data:/data
+    # Override CMD to customise the server name; add --no-guests to require accounts.
+    command: ["--data-dir", "/data", "--name", "My VoiceCat Server"]
+
+volumes:
+  voicecat-data: