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>

cmake/voicecat-toolchain.cmake

@@ -68,5 +68,15 @@ if(NOT DEFINED VCPKG_TARGET_TRIPLET)
     unset(_voicecat_target)
 endif()
 
+# ── Overlay triplets — project-local overrides take precedence ────────────────
+# Enables custom triplets (e.g. release-only x64-linux/arm64-linux, iOS slices)
+# without needing to fork vcpkg's built-in ones.  Cross-compile presets that
+# already set VCPKG_OVERLAY_TRIPLETS explicitly (apple-ios, apple-ios-sim) keep
+# their own value via the DEFINED guard.
+if(NOT DEFINED VCPKG_OVERLAY_TRIPLETS)
+    set(VCPKG_OVERLAY_TRIPLETS "${CMAKE_CURRENT_LIST_DIR}/vcpkg-overlays/triplets"
+        CACHE STRING "vcpkg overlay triplets directory")
+endif()
+
 # ── Hand off to the real vcpkg toolchain ──────────────────────────────────────
 include("$ENV{VCPKG_ROOT}/scripts/buildsystems/vcpkg.cmake")