Add CLAUDE.md (#1137)

<!-- CURSOR_SUMMARY -->
> [!NOTE]
> **Low Risk**
> Documentation-only change with no runtime or behavioral impact.
> 
> **Overview**
> Adds `CLAUDE.md` with contributor workflow notes: preferred dependency
managers (pnpm/poetry), required formatting/lint/typecheck steps, how to
run tests, how to regenerate the API client (`make codegen`), and where
default credentials are stored.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
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<!-- /CURSOR_SUMMARY -->

Author: mishushakov

CLAUDE.md

@@ -0,0 +1,5 @@
+Use pnpm for node and poetry for python to install and update dependencies.
+Run `pnpm run format`, `pnpm run lint` and `pnpm run typecheck` before commiting changes.
+To re-generate the API client run `make codegen` in the repository root.
+Run tests on affected codepaths using `pnpm run test`.
+Default credentials are stored in .env.local in the repository root or inside ~/.e2b/config.json.