feat: exempt matching bots from PR description check

[LuWang1983]

ℹ️ Overview

• skip description validation for bot authors whose login matches alloy or an ai token (hyphen/underscore-delimited), so the check can be made required; bots names are intentionally left out in code due to repo being public
• keep exemptions in-script (not a caller if) so the job reports success instead of staying pending
• other bots must still fill Overview; revert PRs remain exempt

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 implemented bot regex exemption and caller docs; operator directed and reviewed.

🧪 Test Instructions

1. Merge this PR
2. Open a PR authored by a bot whose login contains alloy or an ai token with empty Overview — check should pass
3. Open a Dependabot (or similar third-party bot) PR with empty Overview — check should fail
4. Open a human PR with empty/REPLACE ME Overview — check should still fail
5. 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.

[sunnysingh]

Something to note is if we want to publicize this information (i.e. what are the bot names and machine users we have at Alloy).

We can force push a change to this branch that simply checks the prefix alloy- or suffix -ai of the bot names. Or does a partial match, like *release-orchestrator*. This way it's not exactly known what the names of our bots are.

Alternatively we can just go back to exempting all bots but it's something to consider. Sorry this has had a lot of back-and-forth!