ci: skip CUDA and Metal jobs on pull requests

[gaetan-puleo]

Overview

This PR disables CUDA and Metal build jobs for pull request events while preserving them for pushes to master and manual workflow dispatches.

The goal is to avoid unrelated CUDA and Apple Metal failures blocking Strix Halo ROCm/Vulkan pull requests.

The following PR jobs are skipped:

• CUDA Ubuntu
• macOS ARM64 Metal
• iOS Metal
• tvOS Metal
• visionOS Metal
• Swift Metal builds

The following checks remain enabled on pull requests:

• HIP
• MUSA
• macOS x64 with Metal disabled
• Other existing CPU, Vulkan, formatting, and quality workflows

The workflows are not deleted. CUDA and Metal coverage continues to run on master and through workflow_dispatch.

Measurements

Not applicable. This PR changes GitHub Actions event conditions only and cannot affect runtime performance, model output, or memory use.

Additional information

Each affected job uses:

if: github.event_name != 'pull_request'

This keeps the workflow definitions available without scheduling those jobs for PR events.

What was not verified:

• Repository branch-protection rules that may still list these jobs as required checks
• Manual workflow execution after merge
• CUDA or Metal compilation, because this PR intentionally changes only when those jobs run

Requirements

• I have read and agree with the contributing guidelines
• This change is specific to the Strix Halo fork's CI policy
• AI usage disclosure: AGENT-AUTHORED - OpenCode assisted with implementation, branch preparation, and this PR description
• What was NOT verified: branch-protection configuration and post-merge manual dispatch