fix: respect `!` in grind's instantiate when the theorem is registered

This PR fixes `instantiate only [!thm]` ignoring the `!` modifier when `thm` is already registered as a `grind` theorem, and using its registered patterns instead of the minimal indexable ones. Generated scripts using `!thm` then failed to replay, and `grind?` dropped `thm` from its `grind only` suggestions.

Registered theorems are now reused only if they match the requested `!` modifier.

Closes #16034

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>