Drop macOS x86_64 build target (#55)

Stop producing the omnigraph-macos-x86_64 archive in both the
stable and edge release workflows. The macos-15-intel runner
build was the slowest of the matrix and Apple Silicon is now
the default Mac developer target.

- release.yml + release-edge.yml: drop the macos-15-intel matrix entry
- install.sh: drop the Darwin/x86_64 case so Intel Macs get a clear
  "no prebuilt binary" error instead of attempting an absent download
- update-homebrew-formula.sh: drop the MACOS_X86_* variables and emit
  an arm64-only Homebrew formula. The on_macos block now declares
  `depends_on arch: :arm64` so Intel `brew install` fails fast with
  a clear architecture message instead of installing an arm64 binary
  that errors at exec time.

Linux x86_64 build is unaffected.

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

.github/workflows/release-edge.yml

@@ -41,8 +41,6 @@ jobs:
         include:
           - runner: ubuntu-latest
             asset_name: omnigraph-linux-x86_64
-          - runner: macos-15-intel
-            asset_name: omnigraph-macos-x86_64
           - runner: macos-14
             asset_name: omnigraph-macos-arm64
     env:

.github/workflows/release.yml

@@ -18,8 +18,6 @@ jobs:
         include:
           - runner: ubuntu-latest
             asset_name: omnigraph-linux-x86_64
-          - runner: macos-15-intel
-            asset_name: omnigraph-macos-x86_64
           - runner: macos-14
             asset_name: omnigraph-macos-arm64
     env: