feat: add self-check tip to check output (#2574)

* feat: add self-check tip to check output

* style: drop trailing period from self-check tip

Aligns the new tip with the other `Tip:` lines in `specify check`,
which don't end in a period. Per Copilot review feedback on #2574.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>