Add start gate abort window to publish workflow

[bernardladenthin]

Summary

Adds a configurable "start gate" job to the publish workflow that creates a cancellable abort window before the pipeline begins. This allows developers to cancel the workflow during a grace period after triggering it, before any actual build work starts.

Changes

• New startgate job: A simple job that runs on ubuntu-latest using the startgate GitHub Environment, which has a configurable wait timer (configured in repository Settings → Environments → startgate → Wait timer)
• Updated all build/test jobs: Added needs: startgate dependency to all 10 downstream jobs:
  • crosscompile-linux-x86_64-cuda
  • crosscompile-linux-x86_64
  • crosscompile-linux-aarch64
  • crosscompile-android-aarch64
  • build-macos-arm64-no-metal
  • build-macos-arm64-metal
  • build-windows-x86_64
  • build-windows-x86
  • test-cpp-linux-x86_64
  • test-macos-arm64-metal-15

Implementation Details

The startgate job is a minimal gate that simply echoes a message upon completion. The actual abort window is enforced by GitHub's environment wait timer feature, which pauses job execution until the timer expires or is manually approved. This provides a safety mechanism to catch and cancel unintended workflow runs before expensive cross-compilation and testing jobs begin.

https://claude.ai/code/session_01TGzxSDP2uzsGiJupDsNad1

[claude]

Review Summary

This PR implements a cancellable abort window for the publish workflow using GitHub's Environment wait timer feature. The implementation is sound and follows GitHub Actions best practices.

Strengths:

• Clean, minimal implementation - the startgate job is simple and does exactly what it needs.
• Correct dependency declaration - all 10 build/test jobs properly declare needs: startgate.
• Good documentation - inline comments explain the feature and configuration location.
• No security concerns - job uses no secrets and poses no risk.
• Transitive dependencies handled correctly - Java test jobs inherit the delay without explicit redeclaration.

Operational Requirements:
The feature requires external setup in repository settings:

• Navigate to Settings > Environments > startgate
• Configure a Wait timer (duration in minutes)
• Without this, the gate becomes a no-op

Recommendation: Document this setup requirement in CONTRIBUTING guide so operators know it needs explicit configuration.

No code issues found. Ready to merge pending environment setup verification.