ci(desktop): move macOS build to arcbox self-hosted fleet

[PeronGH]

Summary

• Switch the desktop macOS build matrix entry from blacksmith-6vcpu-macos-26 to arcbox-macos-arm64, routing the job to our self-hosted arm64 macOS runner.
• Linux and Windows matrix entries are unchanged.

Requirements on the host

mac.icon → Assets.car still needs actool >= 26, so the arcbox macOS host must have Xcode 26 installed and selected (sudo xcode-select -s /Applications/Xcode.app or DEVELOPER_DIR set). The prior comment on the line has been updated to make this explicit.

Signing/notarization is unaffected — MACOS_CSC_*, APPLE_API_KEY*, and APPLE_TEAM_ID still come from the release environment.

Test plan

• Verify the arcbox macOS host has xcodebuild -version reporting 26.x.
• Trigger Build Desktop via workflow_dispatch with sign=false and confirm the Build (mac) job runs on the arcbox runner and packages successfully.
• Cut a signed pre-release / dry run through release-desktop.yml with sign=true and confirm codesign + notarization still pass.

[Copilot]

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[greptile-apps]

Greptile Summary

This PR moves the desktop macOS build to the arcbox self-hosted runner. The main changes are:

• Routes the macOS matrix entry to arcbox-macos-arm64.
• Leaves the Linux and Windows matrix entries unchanged.
• Documents that the macOS host needs Xcode 26 for actool to compile mac.icon into Assets.car.

Confidence Score: 5/5

Safe to merge with minimal risk.

The change is limited to a single GitHub Actions matrix runner label and a clarifying comment; the existing build, signing, notarization, and artifact steps remain unchanged.

No files require special attention.

T-Rex Logs

What T-Rex did

• All workflow checks passed for the macOS target, including platform mac, workflow_dispatch sign default false, package invocation, verify invocation, and the unsigned mac CSC_IDENTITY_AUTO_DISCOVERY: false setting.
• The desktop Turborepo dry-run build completed with exit code 0.
• The desktop script syntax checks completed with exit code 0.
• Actionlint ran with configured labels and exited 0, and an unconfigured run reported unknown custom runner labels.
• Linux sandbox tooling was missing for macOS packaging validation, blocking xcodebuild and xcrun based verification.

_{Ran code and verified through T-Rex}

Important Files Changed

Filename	Overview
.github/workflows/build-desktop.yml	Updates only the macOS desktop build matrix runner label and clarifies the Xcode 26 host requirement; no issues found.

_{Reviews (2): Last reviewed commit: "ci(desktop): move macOS build to arcbox ..." | Re-trigger Greptile}