build: bundle vcpkg as a git submodule, pinned to the manifest baseline

Adds vcpkg as a submodule at vcpkg/, pinned to the exact commit vcpkg.json
already declares as builtin-baseline, so the bundled checkout and the
manifest's resolved port versions can never drift apart.

cmake/voicecat-toolchain.cmake, scripts/common.sh, and
clients/apple/scripts/build-xcframework.sh now resolve vcpkg as:
VCPKG_ROOT env var (external checkout) > bundled submodule. Docs updated
to describe the new one-time setup.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

AGENTS.md

@@ -38,15 +38,20 @@ making progress.
 
 ## Build
 
-Default development preset (real deps via vcpkg — works on Windows/Linux/macOS):
+Default development preset (real deps via vcpkg — works on Windows/Linux/macOS). vcpkg is
+bundled as a git submodule at `vcpkg/`, pinned to `vcpkg.json`'s `builtin-baseline`:
 
 ```bash
-export VCPKG_ROOT=/path/to/vcpkg     # bootstrap vcpkg first; cross-platform
+git submodule update --init vcpkg   # one-time, after cloning
+./vcpkg/bootstrap-vcpkg.sh          # .bat on Windows
 cmake --preset dev
 cmake --build --preset dev
 ctest --preset dev
 ```
 
+To use an external vcpkg checkout instead, `export VCPKG_ROOT=/path/to/vcpkg` — it always
+takes priority over the bundled submodule.
+
 Skeleton (no third-party deps — works immediately, no vcpkg needed):
 
 ```bash
@@ -68,7 +73,8 @@ for the full matrix.
 
 `vcpkg.json` pins all deps to a fixed vcpkg baseline — `cmake --preset dev` resolves them
 automatically on first configure. The vcpkg triplet is auto-resolved from the host platform
-by [`cmake/voicecat-toolchain.cmake`](cmake/voicecat-toolchain.cmake).
+by [`cmake/voicecat-toolchain.cmake`](cmake/voicecat-toolchain.cmake), which also resolves
+`VCPKG_ROOT` (env var override, else the bundled `vcpkg/` submodule).
 
 ## Where each subsystem lives (and its doc)