chore(ci): improve CLA instructions (#25039)

* chore(cla): improve bot message with copy-pastable sign instruction

Add a clearer `custom-notsigned-prcomment` that includes the sign phrase
in a code block (one-click copy in GitHub UI) and a note for contributors
whose commit email is not linked to their GitHub account.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* repurpose

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Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>