CI: prune CODEOWNERS avalanche on non-draft PR open

[brtnfld]

Summary

• When a non-draft PR opens, GitHub immediately auto-assigns every CODEOWNERS owner for all touched areas.
• The previous additive-only logic saw every area as already covered and returned the full auto-assigned set as `confirmedRequested`, so the checklist @-mentioned all owners before the final reviewer set was known (e.g. PR CI: base draft PR staleness on real activity, not pr.updated_at #6472 got all four `.github` owners pinged).
• On `opened`/`reopened` (non-draft), now mirrors the draft behavior: passes an empty `existingRequested` to `chooseReviewers` to get a fresh load-balanced pick, then calls `removeUnselected` to prune everyone GitHub auto-assigned who isn't in that pick. The checklist is posted with only the selected reviewer(s).
• Also folds in the bot-sender guard (previously PR CI: guard reviewer-exclusion update against bot-triggered removals #6473, now closed): the `removeUnselected` calls fire `review_request_removed` events that self-trigger new workflow runs. Without the guard, those runs would record the bot's own removals as deliberate human decisions and permanently add the pruned logins to the exclusion set, blocking them from ever being auto-assigned to the PR again.

[github-actions]

Review Checklist

This PR touches the following areas. Each needs a sign-off
from its listed owners before merging.

• .github ✅ — @lrknox

✅ All areas have been signed off.