Add fork-compatible smoke-test workflow and visibility diagnosis

[Copilot]

GitHub Actions workflows don't appear in fork Actions tabs because 14 of 18 workflows use triggers incompatible with forks (pull_request_target, issues, schedule, upstream branch pushes).

Changes

• smoke-test.yml: Minimal workflow_dispatch workflow for verifying Actions are enabled in forks

  • Can be manually triggered from Actions tab
  • Validates checkout, basic commands, and provides clear success feedback

• GITHUB_ACTIONS_FORK_DIAGNOSIS.md: Technical breakdown of workflow compatibility

  • Categorizes all 18 workflows by fork compatibility
  • Documents which triggers work/don't work in forks
  • Provides fork user instructions

Context

Fork-incompatible triggers by count:

• pull_request_target: 4 workflows (upstream PRs only)
• issues: 4 workflows (events don't propagate to forks)
• schedule: 2 workflows (disabled in forks)
• Upstream branch restrictions: 4+ workflows

Only 5 existing workflows have workflow_dispatch. This adds a sixth specifically designed as a diagnostic tool.

Original prompt

Investigate why GitHub Actions workflows are not visible in the user's fork Actions tab. Provide a clear diagnosis and steps to fix. Check repository settings, fork behavior, workflow file presence on default branch, workflow validity (path/extension/on triggers), reusable-workflow-only cases, permissions/policies, and fork sync status. If needed, propose minimal change (e.g., add workflow_dispatch smoke test) to verify Actions registration, but only if appropriate.

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[mergify]

This pull request does not have a backport label. Could you fix it @Copilot? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.
• If no backport is necessary, please add the backport-skip label