remove examples to be absorved as templates (#753)

🪓 

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Removes legacy example projects to be replaced by templates.
> 
> - Deletes entire `contrib/examples/*` subtrees: `ai-sdk`, `crewai`,
`langchain`, `langchain-ts`, `langgraph-ts`, and `mastra`
> - Removes associated source files, READMEs, env samples, package/lock
files, and miscellaneous configs
> 
> No product code changed; docs/examples only.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
0efc4ec. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: torresmateo