Enable automerge with better failure comments

[mattleibow]

Two changes:

1. Enable automerge — flips DRY_RUN to false

2. Better failure comments — the comment now distinguishes between:

   • Validation failures: new warnings, check failures (shows the reason)
   • Merge failures: branch not up to date, SHA mismatch, etc. (shows the gh error)
   • Both together: if validation passes but merge fails, shows both

Example for the 'not up to date' case:

⚠️ Auto-merge blocked

Merge failed: Pull request is not mergeable: the head branch is not up to date with the base branch.
The PR branch needs to be updated with main before it can be merged.

To revert automerge: change DRY_RUN: false back to DRY_RUN: true.

[learn-build-service-prod]

Learn Build status updates of commit d8ef5cd:

✅ Validation status: passed

File	Status	Preview URL	Details
.github/workflows/automerge-docs.yml	✅Succeeded

For more details, please refer to the build report.

[learn-build-service-prod]

PoliCheck Scan Report

The following report lists PoliCheck issues in PR files. Before you merge the PR, you must fix all severity-1 and severity-2 issues. The AI Review Details column lists suggestions for either removing or replacing the terms. If you find a false positive result, mention it in a PR comment and include this text: #policheck-false-positive. This feedback helps reduce false positives in future scans.

✅ No issues found

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