fix: playground docs

[mrpaaradox]

Few weeks ago there was a change in the Playground docs where instead of (agent-playground) only (agent) was selected to import the definePlaygroundAPI. Previous version of the docs was working fine. Hence I am making this PR to reverse that change.

I have tested agent package & I couldn't find any supporting changing which can help us import the definePlaygroundAPI.
Also I see no updates for the same on the Official Convex Playground page as shown below.

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[nipunn1313]

@ianmacartney - can you review? If it looks good, I can merge.

[ianmacartney]

@mrpaaradox as of version 0.1.17 (I believe) the Agent exports the definePlaygroundAPI function directly and is generally the pattern I recommend. That's a good callout that the hosted playground is outdated - I'm making that change to update it there to the new syntax. If you have any issues importing it from the Agent directly please let me know - filing a GitHub issue in the Agent repo would be preferred: https://github.com/get-convex/agent