CI: Document edge cases for /run-skipped-ci workflow

[justin808]

Background

The /run-skipped-ci workflow (from PR #1995) has some edge cases that should be documented for users.

Edge Cases to Document

1. Force-Pushed PRs

Issue: If someone force-pushes after commenting /run-skipped-ci, the workflows will run against the old SHA, potentially confusing users.

Example scenario:

1. User comments /run-skipped-ci on commit ABC123
2. User immediately force-pushes new commit DEF456
3. Full CI runs against ABC123 (old commit)
4. User is confused why their new changes aren't being tested

Mitigation options:

• Document this behavior in CONTRIBUTING.md
• Add a check in the workflow to verify the PR head SHA hasn't changed
• Display a warning if SHA mismatch is detected

2. Deleted Branches

Issue: If a PR's branch is deleted while workflows are running, they'll fail with unclear errors.

Example scenario:

1. User comments /run-skipped-ci
2. User force-pushes and branch ref changes
3. Workflows fail to checkout code

Mitigation:

• Document that workflows will fail if branch is deleted/force-pushed during execution
• Suggest waiting for workflows to complete before force-pushing

Proposed Documentation

Add to CONTRIBUTING.md under the /run-skipped-ci section:

### Important Notes

- **Force-pushes:** The workflow runs against the commit SHA at the time of the comment. If you force-push after commenting, the workflow will test the old commit. Comment again to test the new commit.
- **Branch operations:** Avoid deleting or force-pushing branches while workflows are running, as this may cause failures.

Priority

Low - These are edge cases that rarely occur in practice, but documentation would help prevent confusion.

Related

• PR Improve Streamed Components retry logic #1995: CI comment trigger security and reliability improvements
• Issue CI: Fix workflow matrix exclusion logic with empty strings #1999: Matrix exclusion logic improvements
• Issue CI: Improve workflow verification retry logic in run-skipped-ci #2000: Retry logic improvements