feat: skip PR description check for bot-authored PRs

[LuWang1983]

ℹ️ Overview

• skip description validation for bot-authored PRs (user.type === 'Bot') so the check can be made required without blocking Dependabot/etc.
• keep the exemption in-script (not a caller if) so the job still reports success instead of staying pending
• document that callers must always invoke the callable workflow

Relates to INFR-6089.

🤖 Coding AI Authorship

• Human-authored: written almost entirely by hand; maybe some autocomplete, sourced code or scaffolding.
• Co-authored: agent wrote meaningful portions, significantly edited by me.
• AI-authored: nearly all agent-authored with minimal human edits if any.
  AI added bot skip + caller docs; operator directed and reviewed.

🧪 Test Instructions

1. Merge this PR
2. Open a bot-authored PR (or simulate user.type === Bot payload) against a repo calling this workflow — check should pass without Overview content
3. Open a human PR with empty/REPLACE ME Overview — check should still fail
4. Confirm callers do not gate the job with if: user.type != 'Bot'

Test Results

Not yet run — pending merge.

📝 Authoring Guidelines

As the author, I verify that I have:

• Followed the test instructions and updated the test results.
• Added/updated unit tests as applicable.
• Added/updated documentation as applicable.

🚨 Risks

• Other core setup (please clarify below)

Required-check behavior depends on callers always invoking this workflow. A caller-level skip would leave bot PRs pending.

[Jarvis-Alloy]

Related Jira Issue: INFR-6089 Improve pr-description-check GitHub Actions workflow.