chore(ci): drop unused brew install + ORT_DYLIB_PATH from CI steps

Build is a cross-compile (macos-latest runner is Apple Silicon targeting
x86_64-apple-darwin) and ort-load-dynamic doesn't link libonnxruntime at
build time — only at runtime via dlopen. So the brew install step and
ORT_DYLIB_PATH export were ceremony without payload. Removed to cut CI
time. Runtime setup remains documented in docs/INSTALL-INTEL-MAC.md for
end users installing the tarball on their own Intel Mac.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.github/workflows/release.yml

@@ -42,12 +42,10 @@ jobs:
             os: macos-latest
             archive: tar.gz
             cargo_flags: "--no-default-features --features ort-dynamic,vector-search"
-            needs_onnxruntime: true
           - target: aarch64-apple-darwin
             os: macos-latest
             archive: tar.gz
             cargo_flags: ""
-            needs_onnxruntime: false
 
     steps:
       - name: Checkout
@@ -58,12 +56,6 @@ jobs:
         with:
           targets: ${{ matrix.target }}
 
-      - name: Install ONNX Runtime (Intel Mac dynamic link)
-        if: matrix.needs_onnxruntime
-        run: |
-          brew install onnxruntime
-          echo "ORT_DYLIB_PATH=$(brew --prefix onnxruntime)/lib/libonnxruntime.dylib" >> $GITHUB_ENV
-
       - name: Build
         run: cargo build --package vestige-mcp --release --target ${{ matrix.target }} ${{ matrix.cargo_flags }}