ci: update PR title validation to accept revert titles and move skip changelog to description

[BigLep]

Summary

• Accept "Revert ..." titles from GitHub's revert functionality in pr-title-check.yml
• Remove [skip changelog] prefix support from PR titles
• Update changelog.yml to check for [skip changelog] in PR description instead of title
• Update CONTRIBUTING.md to document both changes

Test plan

• Test PR with "Revert ..." title (should pass validation)
• Test PR with "[skip changelog]" in title (should fail validation)
• Test PR with "[skip changelog]" in description (should skip changelog requirement)
• Verify CONTRIBUTING.md documentation matches enforcement logic

Additional notes

• I was triggered to do this as result of Revert "feat(ci): only markdown action to skip some CI workflows" #13164 getting hung up by PR title checks.
• 🤖 Generated with Claude Code

[Copilot]

Pull Request Overview

This PR updates the PR title validation to accept GitHub revert titles and moves the “[skip changelog]” check from the title to the PR description. Key changes include:

• Updating the regex in pr-title-check.yml to accept “Revert ..." titles and remove the “[skip changelog]” prefix support from PR titles.
• Modifying changelog.yml to look for “[skip changelog]” in the PR description instead.
• Updating CONTRIBUTING.md and related workflow documents to reflect these changes.

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
CONTRIBUTING.md	Updated documentation for title conventions and changelog management instructions.
.github/workflows/pr-title-check.yml	Updated title validation regex to support GitHub revert titles and remove skip changelog support.
.github/workflows/changelog.yml	Changed condition to check the PR description for “[skip changelog]” instead of the title.
.github/workflows/CLAUDE.md	Added reference to AGENTS.md.
.github/workflows/AGENTS.md	Added documentation for validation workflows with a minor markdown syntax issue.

Comments suppressed due to low confidence (1)

.github/workflows/AGENTS.md:6

• There appears to be an extra closing bracket in the markdown link. Please remove the stray ']' after CONTRIBUTING.md to ensure correct formatting.

- **CRITICAL**: Changes to validation logic MUST be reflected in CONTRIBUTING.md](../../CONTRIBUTING.md).  CONTRIBUTING.md and enforcement workflows should be kept in sync.